Change findOwnershipReferenceRoot to stop at phis.

This API should not handle is phis because the phi argument's ownedhip
is independent from the phi itself. The client assumes that the
returned root is in the same lifetime/scope of the access.

Author: atrick

lib/SIL/Utils/MemAccessUtils.cpp

@@ -732,12 +732,8 @@ namespace {
 // Essentially RC identity where the starting point is already a reference.
 class FindReferenceRoot {
   SmallPtrSet<SILPhiArgument *, 4> visitedPhis;
-  bool preserveOwnership;
 
 public:
-  FindReferenceRoot(bool preserveOwnership)
-      : preserveOwnership(preserveOwnership) {}
-
   SILValue findRoot(SILValue ref) && {
     SILValue root = recursiveFindRoot(ref);
     assert(root && "all phi inputs must be reachable");
@@ -749,14 +745,8 @@ class FindReferenceRoot {
   SILValue recursiveFindRoot(SILValue ref) {
     while (auto *svi = dyn_cast<SingleValueInstruction>(ref)) {
       // If preserveOwnership is true, stop at the first owned root
-      if (preserveOwnership) {
-        if (!isIdentityAndOwnershipPreservingRefCast(svi)) {
-          break;
-        }
-      } else {
-        if (!isIdentityPreservingRefCast(svi)) {
-          break;
-        }
+      if (!isIdentityPreservingRefCast(svi)) {
+        break;
       }
       ref = svi->getOperand(0);
     };
@@ -792,11 +782,22 @@ class FindReferenceRoot {
 } // end anonymous namespace
 
 SILValue swift::findReferenceRoot(SILValue ref) {
-  return FindReferenceRoot(false /*preserveOwnership*/).findRoot(ref);
+  return FindReferenceRoot().findRoot(ref);
 }
 
+// This does not handle phis because a phis is either a consume or a
+// reborrow. In either case, the phi argument's ownership is independent from
+// the phi itself. The client assumes that the returned root is in the same
+// lifetime or borrow scope of the access.
 SILValue swift::findOwnershipReferenceRoot(SILValue ref) {
-  return FindReferenceRoot(true /*preserveOwnership*/).findRoot(ref);
+  while (auto *svi = dyn_cast<SingleValueInstruction>(ref)) {
+    if (isIdentityAndOwnershipPreservingRefCast(svi)) {
+      ref = svi->getOperand(0);
+      continue;
+    }
+    break;
+  }
+  return ref;
 }
 
 /// Find the first owned aggregate containing the reference, or simply the