[llvm] [RISCV][CG]Use processShuffleMasks for per-register shuffles (PR #120803)

[Philip Reames via llvm-commits]

preames wrote:

Hm, noticed the usual auto update on the review didn't happen for some reason.  I have revered this in commit 684052173 for the reason described in the revert commit.  

https://github.com/llvm/llvm-project/pull/120803