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

[Mikhail R. Gadelha via llvm-commits]

mikhailramalho wrote:

@preames could you take another look now?

It doesn't include the slidedown you suggested. I figured we could do that in a separate patch, if you don't mind.

I'll also rename the variable once this patch lands, I did that on this PR and all the renaming made the patch a bit more confusing.

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