Swift SIL: replace the set_deallocating instruction with begin_dealloc_ref

Codegen is the same, but `begin_dealloc_ref` consumes the operand and produces a new SSA value.
This cleanly splits the liferange to the region before and within the destructor of a class.

lib/SIL/Utils/MemAccessUtils.cpp

@@ -776,6 +776,7 @@ LLVM_ATTRIBUTE_USED void AccessBase::dump() const { print(llvm::dbgs()); }
 bool swift::isIdentityPreservingRefCast(SingleValueInstruction *svi) {
   // Ignore both copies and other identity and ownership preserving casts
   return isa<CopyValueInst>(svi) || isa<BeginBorrowInst>(svi)
+         || isa<BeginDeallocRefInst>(svi)
          || isIdentityAndOwnershipPreservingRefCast(svi);
 }