[llvm] [RISCV] Recognize de-interleave shuffles with 2 sources. (PR #127272)

[Philip Reames via llvm-commits]

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

LGTM

Nice win, that was a much simpler patch than I'd had as in my head.  

https://github.com/llvm/llvm-project/pull/127272