[Transforms] Fix -Wsign-compare in DemoteRegToStack.cpp (NFC)

llvm-project/llvm/lib/Transforms/Utils/DemoteRegToStack.cpp:54:23:
error: comparison of integers of different signs: 'int' and 'unsigned int' [-Werror,-Wsign-compare]
    for (int i = 0; i < CBI->getNumSuccessors(); i++) {
                    ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.