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