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

[Philip Reames via llvm-commits]

preames wrote:

> For low LMUL it would be better to concat first, but I'm leaving
this for later.

You might want to add deinterleave to isShuffleMaskLegal before adding the custom low LMUL lowering.  I've seen some interactions there before.

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