[llvm] [RISCV] Reduce the LMUL for a vrgather operation if legal (PR #125768)

[Philip Reames via llvm-commits]

preames wrote:

Small observation here: This is placed after the recognition of splat's (i.e. vrgather.vi/x).  This does cause us to miss a few splat cases which could be done at lower LMUL, but I'm no sure how much we care since the splats are generally linear in LMUL anyways.

https://github.com/llvm/llvm-project/pull/125768