[llvm] [RISCV] Decompose LMUL > 1 reverses into LMUL * M1 vrgather.vv (PR #104574)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM

It seems like the profitability discussion has settled out with a "better in some cases, and not significantly worse in others" answer, and thus this is reasonable to enable by default.  Please chime in if I'm off in believing we have consensus this is okay.

On the vectorizer discussion, that seems like an unrelated work item.  Fair warning, I had looked into that a long time ago and it was a more complicated than it first appears.  

https://github.com/llvm/llvm-project/pull/104574