[Analysis] add optional index parameter to isSplatValue()

We want to allow splat value transforms to improve PR44588 and related bugs:
https://bugs.llvm.org/show_bug.cgi?id=44588
...but to do that, we need to know if values are splatted from the same,
specific index (lane) rather than splatted from an arbitrary index.

We can improve the undef handling with 1-liner follow-ups because the
Constant API optionally allow undefs now.

Differential Revision: https://reviews.llvm.org/D73549