[region-isolation] When determining isolation from a class_method, check for global actor isolation first.

Before this change in the following code, we would say that message is isolated to the actor instead of the global actor isolation of the actor's method:

```swift
class Message { ... }

actor MessageHolder {
  @mainactor func hold(_ message: Message) { ... }
}

@mainactor
func sendMessage() async {
    let messageHolder = MessageHolder()
    let message = Message()
    // We identified messageHolder.hold as being MessageHolder isolated
    // instead of main actor isolated.
    messageHolder.hold(message)
    Task { @mainactor in print(message) }
}
```

I also used this as an opportunity to simplify the logic in this part of the
code. Specifically, I had made it so that multiple interesting cases were
handled in the same conditional statement in a manner that it made it hard to
know which cases were actually being handled and why it was correct. Now I split
that into two separate if statements with comments that make it clear what we
are actually trying to pattern match against.

rdar://130980933

Author: gottesmm

include/swift/AST/ActorIsolation.h

@@ -180,6 +180,12 @@ class ActorIsolation {
     return parameterIndex;
   }
 
+  /// Returns true if this actor-instance isolation appllies to the self
+  /// parameter of a method.
+  bool isActorInstanceForSelfParameter() const {
+    return getActorInstanceParameter() == 0;
+  }
+
   bool isSILParsed() const { return silParsed; }
 
   bool isActorIsolated() const {

lib/SILOptimizer/Utils/SILIsolationInfo.cpp

@@ -604,40 +604,55 @@ SILIsolationInfo SILIsolationInfo::get(SILInstruction *inst) {
         bool isNonIsolatedUnsafe = exprAnalysis.hasNonisolatedUnsafe();
         {
           auto isolation = swift::getActorIsolation(dre->getDecl());
-          if (isolation.isActorIsolated() &&
-              (isolation.getKind() != ActorIsolation::ActorInstance ||
-               isolation.getActorInstanceParameter() == 0)) {
-            if (cmi->getOperand()->getType().isAnyActor()) {
+
+          if (isolation.isActorIsolated()) {
+            // Check if we have a global actor and handle it appropriately.
+            if (isolation.getKind() == ActorIsolation::GlobalActor) {
+              bool localNonIsolatedUnsafe =
+                  isNonIsolatedUnsafe | isolation.isNonisolatedUnsafe();
+              return SILIsolationInfo::getGlobalActorIsolated(
+                         cmi, isolation.getGlobalActor())
+                  .withUnsafeNonIsolated(localNonIsolatedUnsafe);
+            }
+
+            // In this case, we have an actor instance that is self.
+            if (isolation.getKind() != ActorIsolation::ActorInstance &&
+                isolation.isActorInstanceForSelfParameter()) {
+              bool localNonIsolatedUnsafe =
+                  isNonIsolatedUnsafe | isolation.isNonisolatedUnsafe();
               return SILIsolationInfo::getActorInstanceIsolated(
-                  cmi, cmi->getOperand(),
-                  cmi->getOperand()
-                      ->getType()
-                      .getNominalOrBoundGenericNominal());
+                         cmi, cmi->getOperand(),
+                         cmi->getOperand()
+                             ->getType()
+                             .getNominalOrBoundGenericNominal())
+                  .withUnsafeNonIsolated(localNonIsolatedUnsafe);
             }
-            return SILIsolationInfo::getGlobalActorIsolated(
-                cmi, isolation.getGlobalActor());
           }
-
-          isNonIsolatedUnsafe |= isolation.isNonisolatedUnsafe();
         }
 
         if (auto type = dre->getType()->getNominalOrBoundGenericNominal()) {
           if (auto isolation = swift::getActorIsolation(type)) {
-            if (isolation.isActorIsolated() &&
-                (isolation.getKind() != ActorIsolation::ActorInstance ||
-                 isolation.getActorInstanceParameter() == 0)) {
-              if (cmi->getOperand()->getType().isAnyActor()) {
+            if (isolation.isActorIsolated()) {
+              // Check if we have a global actor and handle it appropriately.
+              if (isolation.getKind() == ActorIsolation::GlobalActor) {
+                bool localNonIsolatedUnsafe =
+                    isNonIsolatedUnsafe | isolation.isNonisolatedUnsafe();
+                return SILIsolationInfo::getGlobalActorIsolated(
+                           cmi, isolation.getGlobalActor())
+                    .withUnsafeNonIsolated(localNonIsolatedUnsafe);
+              }
+
+              // In this case, we have an actor instance that is self.
+              if (isolation.getKind() != ActorIsolation::ActorInstance &&
+                  isolation.isActorInstanceForSelfParameter()) {
+                bool localNonIsolatedUnsafe =
+                    isNonIsolatedUnsafe | isolation.isNonisolatedUnsafe();
                 return SILIsolationInfo::getActorInstanceIsolated(
                            cmi, cmi->getOperand(),
                            cmi->getOperand()
                                ->getType()
                                .getNominalOrBoundGenericNominal())
-                    .withUnsafeNonIsolated(isNonIsolatedUnsafe);
-              }
-
-              if (auto globalIso = SILIsolationInfo::getGlobalActorIsolated(
-                      cmi, isolation.getGlobalActor())) {
-                return globalIso.withUnsafeNonIsolated(isNonIsolatedUnsafe);
+                    .withUnsafeNonIsolated(localNonIsolatedUnsafe);
               }
             }
           }

test/Concurrency/transfernonsendable.swift

@@ -40,6 +40,7 @@ actor MyActor {
   func useSendableFunction(_: @Sendable () -> Void) {}
   func useNonSendableFunction(_: () -> Void) {}
   func doSomething() {}
+  @MainActor func useKlassMainActor(_ x: NonSendableKlass) {}
 }
 
 final actor FinalActor {
@@ -1809,3 +1810,16 @@ actor FunctionWithSendableResultAndIsolationActor {
         return ""
     }
 }
+
+@MainActor
+func testThatGlobalActorTakesPrecedenceOverActorIsolationOnMethods() async {
+  let a = MyActor()
+  let ns = NonSendableKlass()
+
+  // 'ns' should be main actor isolated since useKlassMainActor is @MainActor
+  // isolated. Previously we would let MyActor take precedence here...
+  a.useKlassMainActor(ns)
+
+  // Meaning we would get an error here.
+  Task { @MainActor in print(ns) }
+}