Fix range-loop-analysis warning

```
llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:8024:19: warning: loop variable 'VF' of type 'const llvm::ElementCount' creates a copy from type 'const llvm::ElementCount' [-Wrange-loop-analysis]
  for (const auto VF : VFCandidates) {
                  ^
llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:8024:8: note: use reference type 'const llvm::ElementCount &' to prevent copying
  for (const auto VF : VFCandidates) {
       ^~~~~~~~~~~~~~~
                  &
1 warning generated.
```

Differential Revision: https://reviews.llvm.org/D103970