[llvm] [RISCV] Merge shuffle sources if lanes are disjoint (PR #119401)

[Philip Reames via llvm-commits]

preames wrote:

Thought of a possible follow on extension - noting it for future reference.

In addition to the select case, we can merge the masks through other cheap (linear in LMUL) instructions.  The two that occur are vslideup.vi and vslidedown.vi, but we might be able to apply other shuffle techniques (i.e. interleave).  The vslidedown.vi subcase is potentially useful for e.g. deinterleave(N) where N is even.  

https://github.com/llvm/llvm-project/pull/119401