[embedded] Compile-time (literal) KeyPaths for Embedded Swift

Enable KeyPath/AnyKeyPath/PartialKeyPath/WritableKeyPath in Embedded Swift, but
for compile-time use only:

- Add keypath optimizations into the mandatory optimizations pipeline
- Allow keypath optimizations to look through begin_borrow, to make them work
  even in OSSA.
- If a use of a KeyPath doesn't optimize away, diagnose in PerformanceDiagnostics
- Make UnsafePointer.pointer(to:) transparent to allow the keypath optimization
  to happen in the callers of UnsafePointer.pointer(to:).

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/CMakeLists.txt

@@ -21,6 +21,7 @@ swift_compiler_sources(Optimizer
   SimplifyDestructure.swift
   SimplifyGlobalValue.swift
   SimplifyInitEnumDataAddr.swift
+  SimplifyKeyPath.swift
   SimplifyLoad.swift
   SimplifyPartialApply.swift
   SimplifyPointerToAddress.swift

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyApply.swift

@@ -17,6 +17,10 @@ extension ApplyInst : OnoneSimplifyable {
     if tryTransformThickToThinCallee(of: self, context) {
       return
     }
+    if context.tryOptimizeKeypath(apply: self) {
+      context.erase(instruction: self)
+      return
+    }
     _ = context.tryDevirtualize(apply: self, isMandatory: false)
   }
 }

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyKeyPath.swift

@@ -0,0 +1,35 @@
+//===--- SimplifyKeyPath.swift --------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SIL
+
+extension KeyPathInst : OnoneSimplifyable {
+  func simplify(_ context: SimplifyContext) {
+    if allUsesRemovable(instruction: self) {
+      context.erase(instructionIncludingAllUsers: self)
+    }
+  }
+}
+
+fileprivate func allUsesRemovable(instruction: Instruction) -> Bool {
+  for result in instruction.results {
+    for use in result.uses {
+      if !(use.instruction is UpcastInst || use.instruction is DestroyValueInst || use.instruction is BeginBorrowInst || use.instruction is EndBorrowInst) {
+        return false
+      }
+      if !allUsesRemovable(instruction: use.instruction) {
+        return false
+      }
+    }
+  }
+  return true
+}

SwiftCompilerSources/Sources/Optimizer/PassManager/Context.swift

@@ -170,6 +170,10 @@ extension MutatingContext {
     }
     return nil
   }
+  
+  func tryOptimizeKeypath(apply: FullApplySite) -> Bool {
+    return _bridged.tryOptimizeKeypath(apply.bridged)
+  }
 
   func inlineFunction(apply: FullApplySite, mandatoryInline: Bool) {
     // This is only a best-effort attempt to notity the new cloned instructions as changed.