SIL, Optimizer: update and handle borrowed-from instructions

Compute, update and handle borrowed-from instruction in various utilities and passes.
Also, used borrowed-from to simplify `gatherBorrowIntroducers` and `gatherEnclosingValues`.
Replace those utilities by `Value.getBorrowIntroducers` and `Value.getEnclosingValues`, which return a lazily computed Sequence of borrowed/enclosing values.

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyBranch.swift

@@ -49,7 +49,15 @@ private extension BranchInst {
     let parentBB = parentBlock
 
     for (argIdx, op) in operands.enumerated() {
-      targetBB.arguments[argIdx].uses.replaceAll(with: op.value, context)
+      let arg = targetBB.arguments[argIdx]
+      if let phi = Phi(arg),
+         let bfi = phi.borrowedFrom
+      {
+        bfi.uses.replaceAll(with: op.value, context)
+        context.erase(instruction: bfi)
+      } else {
+        arg.uses.replaceAll(with: op.value, context)
+      }
     }
     targetBB.eraseAllArguments(context)
 

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifySwitchEnum.swift

@@ -48,12 +48,14 @@ extension SwitchEnumInst : OnoneSimplifyable {
       precondition(enumInst.payload == nil || !parentFunction.hasOwnership,
                    "missing payload argument in switch_enum case block")
       builder.createBranch(to: caseBlock)
+      context.erase(instruction: self)
     case 1:
       builder.createBranch(to: caseBlock, arguments: [enumInst.payload!])
+      context.erase(instruction: self)
+      updateBorrowedFrom(for: [Phi(caseBlock.arguments[0])!], context)
     default:
       fatalError("case block of switch_enum cannot have more than 1 argument")
     }
-    context.erase(instruction: self)
 
     if canEraseEnumInst {
       context.erase(instructionIncludingDebugUses: enumInst)

SwiftCompilerSources/Sources/Optimizer/PassManager/PassRegistration.swift

@@ -117,6 +117,7 @@ private func registerSwiftPasses() {
   registerPass(rangeDumper, { rangeDumper.run($0) })
   registerPass(runUnitTests, { runUnitTests.run($0) })
   registerPass(testInstructionIteration, { testInstructionIteration.run($0) })
+  registerPass(updateBorrowedFromPass, { updateBorrowedFromPass.run($0) })
 }
 
 private func registerSwiftAnalyses() {
@@ -126,4 +127,5 @@ private func registerSwiftAnalyses() {
 
 private func registerUtilities() {
   registerVerifier()
+  registerBorrowedFromUpdater()
 }