Fixed warning about signed/unsigned comparison

[rampitec]

Summary

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

[arsenm]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

7410

static_cast? The constructory integer casts are weird

[rampitec]

Changed to static_cast.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

for (SmallVector<Value *, 4>::iterator IncIt = Incoming.begin(),
       unsigned MaxNumElts = MaxVecRegSize / EltSize;
       if (MaxNumElts < 2) {
         ++IncIt;
         continue;
       }
 
       while (SameTypeIt != E &&
              (*SameTypeIt)->getType() == EltTy &&
-             (SameTypeIt - IncIt) < MaxNumElts) {
+             unsigned(SameTypeIt - IncIt) < MaxNumElts) {
         VisitedInstrs.insert(*SameTypeIt);
         ++SameTypeIt;
       }
 
       // Try to vectorize them.
       unsigned NumElts = (SameTypeIt - IncIt);
       LLVM_DEBUG(dbgs() << "SLP: Trying to vectorize starting at PHIs ("
                         << NumElts << ")\n");