Fixed warning about signed/unsigned comparison

I've got the report clang11 issues signed/unsigned mismatch
warning here. For some reason only clang11 seems to issue
this warning.

Differential Revision: https://reviews.llvm.org/D83916

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -7407,7 +7407,7 @@ bool SLPVectorizerPass::vectorizeChainsInBlock(BasicBlock *BB, BoUpSLP &R) {
 
       while (SameTypeIt != E &&
              (*SameTypeIt)->getType() == EltTy &&
-             (SameTypeIt - IncIt) < MaxNumElts) {
+             static_cast<unsigned>(SameTypeIt - IncIt) < MaxNumElts) {
         VisitedInstrs.insert(*SameTypeIt);
         ++SameTypeIt;
       }