Fix AccessUtils Value.referenceRoot to handle ForwardingInstructions.

This fixes all analyses that use AccessUtils in the presence of new
forwarding instructions. This is also needed for consistency with the
C++ source base. And for general sanity.

Utilities that walk the use-def chain should never hard-code a list of
opcodes that happened to work with some specific pass. Passes should
never assume that a basic SIL utility handles specific SIL operations
a certain way. Instead, the utility needs to be defined in terms of
SIL semantics. In this case, the utility is supposed to handle all
instructions that semantically forward ownership of a reference.

Author: atrick

SwiftCompilerSources/Sources/SIL/Utilities/AccessUtils.swift

@@ -546,20 +546,16 @@ extension Value {
   public var referenceRoot: Value {
     var value: Value = self
     while true {
-      switch value {
-      case is BeginBorrowInst, is CopyValueInst, is MoveValueInst,
-           is EndInitLetRefInst,
-           is BeginDeallocRefInst,
-           is UpcastInst, is UncheckedRefCastInst, is EndCOWMutationInst:
-        value = (value as! Instruction).operands[0].value
-      case let mvr as MultipleValueInstructionResult:
-        guard  let bcm = mvr.parentInstruction as? BeginCOWMutationInst else {
-          return value
-        }
-        value = bcm.instance
-      default:
-        return value
+      if let forward = value.forwardingInstruction, forward.preservesIdentity,
+         let operand = forward.singleForwardedOperand {
+        value = operand.value
+        continue
+      }
+      if let transition = value.definingInstruction as? OwnershipTransitionInstruction {
+        value = transition.operand.value
+        continue
       }
+      return value
     }
   }
 }