[NFC] OSSACanonicalizeOwned: Shadow member.

Add a local variable that shadows the `defUseWorklist` instance member,
enabling further constraints (e.g. `const`ness) to eventually be imposed.

Author: nate-chandler

lib/SILOptimizer/Utils/CanonicalizeOSSALifetime.cpp

@@ -1149,6 +1149,9 @@ void CanonicalizeOSSALifetime::rewriteCopies(
     SmallVectorImpl<DestroyValueInst *> const &newDestroys) {
   assert(getCurrentDef()->getOwnershipKind() == OwnershipKind::Owned);
 
+  // Shadow defUseWorklist in order to constrain its uses.
+  auto &defUseWorklist = this->defUseWorklist;
+
   InstructionSetVector instsToDelete(getCurrentDef()->getFunction());
 
   // Visit each operand in the def-use chain.