ReduxWidth check for 0 (llvm#123257)

Added assert to check for underflow of ReduxWidth

	modified:   llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp


Source code analysis flagged the operation (ReduxWwidth - 1) as
potential underflow, since ReduxWidth is unsigned.
Realize that this should never happen if everything is working right,
but added an assert to check for it just in case.

Author: gchaltas

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -20095,6 +20095,7 @@ class HorizontalReduction {
         NumRegs =
             TTI.getNumberOfRegisters(TTI.getRegisterClassForType(true, Tp));
         while (NumParts > NumRegs) {
+          assert(ReduxWidth > 0 && "ReduxWidth is unexpectedly 0.");
           ReduxWidth = bit_floor(ReduxWidth - 1);
           VectorType *Tp = getWidenedType(ScalarTy, ReduxWidth);
           NumParts = TTI.getNumberOfParts(Tp);