Fix MSVC signed/unsigned mismatch warning

Author: RKSimon

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -17000,7 +17000,7 @@ void BoUpSLP::computeMinimumValueSizes() {
     // Check if the root is trunc and the next node is gather/buildvector, then
     // keep trunc in scalars, which is free in most cases.
     if (E.isGather() && IsTruncRoot && E.UserTreeIndices.size() == 1 &&
-        E.Idx > (IsStoreOrInsertElt ? 2 : 1) &&
+        E.Idx > (IsStoreOrInsertElt ? 2u : 1u) &&
         all_of(E.Scalars, [&](Value *V) {
           return V->hasOneUse() || isa<Constant>(V) ||
                  (!V->hasNUsesOrMore(UsesLimit) &&