SimplifyApply: optimize thick calls where the callee is a thin_to_thick_function

```
   %2 = thin_to_thick_function %1
   %3 = apply %2(...) : @callee_guaranteed
 ->
   %2 = thin_to_thick_function %1
   %3 = apply %1(...): @convention(thin)
```

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyApply.swift

@@ -14,6 +14,9 @@ import SIL
 
 extension ApplyInst : OnoneSimplifyable {
   func simplify(_ context: SimplifyContext) {
+    if tryTransformThickToThinCallee(of: self, context) {
+      return
+    }
     _ = context.tryDevirtualize(apply: self, isMandatory: false)
   }
 }
@@ -29,3 +32,29 @@ extension BeginApplyInst : OnoneSimplifyable {
     _ = context.tryDevirtualize(apply: self, isMandatory: false)
   }
 }
+
+/// Optimizes a thick function call if the callee is a `thin_to_thick_function` instruction:
+///
+///   %2 = thin_to_thick_function %1
+///   %3 = apply %2(...) : @callee_guaranteed
+/// ->
+///   %2 = thin_to_thick_function %1
+///   %3 = apply %1(...): @convention(thin)
+///
+private func tryTransformThickToThinCallee(of apply: ApplyInst, _ context: SimplifyContext) -> Bool {
+  if let tttf = apply.callee as? ThinToThickFunctionInst,
+     !apply.callee.type.isCalleeConsumedFunction
+  {
+    let builder = Builder(before: apply, context)
+    let newApply = builder.createApply(function: tttf.operand.value,
+                                       apply.substitutionMap,
+                                       arguments: Array(apply.arguments),
+                                       isNonThrowing: apply.isNonThrowing,
+                                       isNonAsync: apply.isNonAsync,
+                                       specializationInfo: apply.specializationInfo)
+    apply.uses.replaceAll(with: newApply, context)
+    context.erase(instruction: apply)
+    return true
+  }
+  return false
+}

test/IRGen/big_types_corner_cases.sil

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -I %S/Inputs/abi %s -emit-ir | %FileCheck %s
+// RUN: %target-swift-frontend -Xllvm -sil-disable-pass=simplification -I %S/Inputs/abi %s -emit-ir | %FileCheck %s
 
 // REQUIRES: CPU=x86_64
 // REQUIRES: OS=macosx

test/SILGen/reabstract.swift

@@ -1,6 +1,6 @@
 
 // RUN: %target-swift-emit-silgen -module-name reabstract -Xllvm -sil-full-demangle %s | %FileCheck %s
-// RUN: %target-swift-emit-sil -module-name reabstract -Xllvm -sil-full-demangle %s | %FileCheck %s --check-prefix=MANDATORY
+// RUN: %target-swift-emit-sil -module-name reabstract -Xllvm -sil-disable-pass=simplification -Xllvm -sil-full-demangle %s | %FileCheck %s --check-prefix=MANDATORY
 
 func closureTakingOptional(_ fn: (Int?) -> ()) {}
 closureTakingOptional({ (_: Any) -> () in })

test/SILGen/rethrows.swift

@@ -1,5 +1,5 @@
 
-// RUN: %target-swift-emit-sil -module-name rethrows -verify %s | %FileCheck %s
+// RUN: %target-swift-emit-sil -module-name rethrows -Xllvm -sil-disable-pass=simplification -verify %s | %FileCheck %s
 
 @discardableResult
 func rethrower(_ fn: () throws -> Int) rethrows -> Int {

test/SILOptimizer/simplify_apply.sil

@@ -13,6 +13,20 @@ class Bar {
   func foo() -> Int
 }
 
+sil @cl : $@convention(thin) () -> Int
+
+// CHECK-LABEL: sil [ossa] @thick_to_thin :
+// CHECK:         [[F:%.*]] = function_ref @cl
+// CHECK:         apply [[F]]() : $@convention(thin
+// CHECK:       } // end sil function 'thick_to_thin'
+sil [ossa] @thick_to_thin : $@convention(thin) () -> Int {
+bb0:
+  %0 = function_ref @cl : $@convention(thin) () -> Int
+  %1 = thin_to_thick_function %0 : $@convention(thin) () -> Int to $@callee_guaranteed () -> Int
+  %2 = apply %1() : $@callee_guaranteed () -> Int
+  return %2 : $Int
+}
+
 // CHECK-LABEL: sil @devirt_class_method :
 // CHECK:         [[F:%.*]] = function_ref @bar_foo
 // CHECK:         apply [[F]]