[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.

llvm/lib/Transforms/Utils/DemoteRegToStack.cpp

@@ -51,7 +51,7 @@ AllocaInst *llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads,
       (void)BB;
     }
   } else if (CallBrInst *CBI = dyn_cast<CallBrInst>(&I)) {
-    for (int i = 0; i < CBI->getNumSuccessors(); i++) {
+    for (unsigned i = 0; i < CBI->getNumSuccessors(); i++) {
       auto *Succ = CBI->getSuccessor(i);
       if (!Succ->getSinglePredecessor()) {
         assert(isCriticalEdge(II, i) && "Expected a critical edge!");