Add LifetimeDependence utilities

    - LifetimeDependence

    - VariableIntroducerUseDefWalker

    - LifetimeDependenceUseDefWalker

    - LifetimeDependenceDefUseWalker

Author: atrick

SwiftCompilerSources/Sources/Optimizer/Utilities/CMakeLists.txt

@@ -13,6 +13,7 @@ swift_compiler_sources(Optimizer
   Devirtualization.swift
   EscapeUtils.swift
   ForwardingUtils.swift
+  LifetimeDependenceUtils.swift
   OptUtils.swift
   OwnershipLiveness.swift
   SSAUpdater.swift

SwiftCompilerSources/Sources/Optimizer/Utilities/ForwardingUtils.swift

@@ -252,8 +252,8 @@ extension ForwardingDefUseWalker {
     if !needWalk(for: value) { return .continueWalk }
 
     var hasUse = false
-    for operand in value.uses where !operand.isTypeDependent {
-      if walkDown(operand: operand) == .abortWalk {
+    for use in value.uses where !use.isTypeDependent {
+      if walkDown(operand: use) == .abortWalk {
         return .abortWalk
       }
       hasUse = true
@@ -270,7 +270,12 @@ extension ForwardingDefUseWalker {
 
   mutating func walkDownDefault(forwarding operand: Operand) -> WalkResult {
     if let inst = operand.instruction as? ForwardingInstruction {
-      return inst.forwardedResults.walk { walkDownUses(of: $0, using: operand) }
+      let singleOper = inst.singleForwardedOperand
+      if singleOper == nil || singleOper! == operand {
+        return inst.forwardedResults.walk {
+          walkDownUses(of: $0, using: operand)
+        }
+      }
     }
     if let phi = Phi(using: operand) {
       return walkDownUses(of: phi.value, using: operand)