Add BorrowedFromInst::isReborrow.

borrowed-from instructions have different ownership semantics if the are
attached to reborrows vs. guaranteed forwarding phis.

Author: atrick

include/swift/SIL/SILInstruction.h

@@ -4782,6 +4782,8 @@ class BorrowedFromInst final : public InstructionBaseWithTrailingOperands<
   OperandValueArrayRef getEnclosingValues() const {
     return OperandValueArrayRef(getEnclosingValueOperands());
   }
+
+  bool isReborrow() const;
 };
 
 inline auto BeginBorrowInst::getEndBorrows() const -> EndBorrowRange {

lib/SIL/IR/SILInstruction.cpp

@@ -301,6 +301,14 @@ Operand *BeginBorrowInst::getSingleNonEndingUse() const {
   return singleUse;
 }
 
+bool BorrowedFromInst::isReborrow() const {
+  // The forwarded operand is either a phi or undef.
+  if (auto *phi = dyn_cast<SILPhiArgument>(getBorrowedValue())) {
+    return phi->isReborrow();
+  }
+  return false;
+}
+
 namespace {
 class InstructionDestroyer
     : public SILInstructionVisitor<InstructionDestroyer> {