[ownership] Change BorrowingOperand to no longer be convertable to Operand * and provide a conversion to bool instead.

A few notes:

1. We already have provide an operator* that does this and even better makes the
   conversion explicit in the source.

2. The bool operator checks both that kind is not invalid and that the operand
   is non-null. This ensures we can use an `if` to properly check for a valid
   BorrowingOperand.

Author: gottesmm

include/swift/SIL/OwnershipUtils.h

@@ -189,13 +189,13 @@ struct BorrowingOperand {
 
   // A set of operators so that a BorrowingOperand can be used like a normal
   // operand in a light weight way.
-  operator const Operand *() const { return op; }
-  operator Operand *() { return op; }
   const Operand *operator*() const { return op; }
   Operand *operator*() { return op; }
   const Operand *operator->() const { return op; }
   Operand *operator->() { return op; }
 
+  operator bool() const { return kind != BorrowingOperandKind::Invalid && op; }
+
   /// If \p op is a borrow introducing operand return it after doing some
   /// checks.
   static BorrowingOperand get(Operand *op) {

lib/SILOptimizer/Utils/OwnershipOptUtils.cpp

@@ -546,7 +546,7 @@ static void eliminateReborrowsOfRecursiveBorrows(
     // Then check if in our destination block, we have further reborrows. If we
     // do, we need to recursively process them.
     auto *borrowedArg =
-        const_cast<SILPhiArgument *>(bi->getArgForOperand(borrowingOperand));
+        const_cast<SILPhiArgument *>(bi->getArgForOperand(*borrowingOperand));
     auto *baseArg =
         insertOwnedBaseValueAlongBranchEdge(bi, innerCopy, callbacks);
     baseBorrowedValuePair.emplace_back(baseArg, borrowedArg);
@@ -574,7 +574,7 @@ static void eliminateReborrowsOfRecursiveBorrows(
       // edge and undef along all other edges.
       auto borrowingOp = BorrowingOperand::get(use);
       auto *brInst = cast<BranchInst>(borrowingOp.op->getUser());
-      auto *newBorrowedPhi = brInst->getArgForOperand(borrowingOp);
+      auto *newBorrowedPhi = brInst->getArgForOperand(*borrowingOp);
       auto *newBasePhi =
           insertOwnedBaseValueAlongBranchEdge(brInst, baseArg, callbacks);
       baseBorrowedValuePair.emplace_back(newBasePhi, newBorrowedPhi);
@@ -639,7 +639,7 @@ rewriteReborrows(SILValue newBorrowedValue,
       // edge and undef along all other edges.
       auto borrowingOp = BorrowingOperand::get(use);
       auto *brInst = cast<BranchInst>(borrowingOp.op->getUser());
-      auto *newBorrowedPhi = brInst->getArgForOperand(borrowingOp);
+      auto *newBorrowedPhi = brInst->getArgForOperand(*borrowingOp);
       auto *newBasePhi =
           insertOwnedBaseValueAlongBranchEdge(brInst, baseArg, callbacks);
       baseBorrowedValuePair.emplace_back(newBasePhi, newBorrowedPhi);