[llvm] [RISCV] Update matchSplatAsGather to use the index of extract_elt if in-bounds (PR #118873)

[Philip Reames via llvm-commits]

https://github.com/preames requested changes to this pull request.

Gah, accidentally hit "Approve" instead of "Request changes".  I do want to see the rebase and revised version before this lands.  I usually wouldn't, but given the history of this review, I do.  Hopefully finally round will be quick.  

https://github.com/llvm/llvm-project/pull/118873