[SLPVectorizer] Do not assume extracelement idx is a ConstantInt.

The index of an ExtractElementInst is not guaranteed to be a
ConstantInt. It can be any integer value. Check explicitly for
ConstantInts.

The new test cases illustrate scenarios where we crash without
this patch. I've also added another test case to check the matching
of extractelement vector ops works.

Reviewers: RKSimon, ABataev, dtemirbulatov, vporpo

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D74758