[region-isolation] Fix thinko.

The thinko was that rather than use the actual isolated parameter of a partial apply as the isolated parameter, I instead just used the last operand... :doh:. This fixes:

rdar://126297097

Author: gottesmm

lib/SILOptimizer/Utils/PartitionUtils.cpp

@@ -105,7 +105,7 @@ SILIsolationInfo SILIsolationInfo::get(SILInstruction *inst) {
           for (auto &op : as.getArgumentOperands()) {
             if (as.getArgumentParameterInfo(op).hasOption(
                     SILParameterInfo::Isolated)) {
-              actorInstance = pai->getAllOperands().back().get();
+              actorInstance = op.get();
               break;
             }
           }

test/Concurrency/transfernonsendable.swift

@@ -1617,3 +1617,12 @@ extension FinalActor {
     // expected-complete-warning @-2 {{passing argument of non-sendable type 'NonSendableKlass' into actor-isolated context may introduce data races}}
   }
 }
+
+actor DictionaryActorTest {
+  var data: [Int: Int] = [:]
+
+  // We used to crash on this due to isolation merging.
+  func doSomething(_ key: Int) {
+    assert(self.data[key] == 0)
+  }
+}