[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 approved this pull request.

Please rebase and address style comment.  Close to LGTM.

For context of anyone reading along, this could be further improved to catch more cases, but in the goal of encouraging iteration, I'm going to accept the patch once it's functionally correct and catches some additional cases.  

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