[Transforms] Fix -Wunused-variable in DemoteRegToStack.cpp (NFC)

llvm-project/llvm/lib/Transforms/Utils/DemoteRegToStack.cpp:58:21:
error: unused variable 'BB' [-Werror,-Wunused-variable]
        BasicBlock *BB = SplitCriticalEdge(II, i);
                    ^
1 error generated.