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

[Philip Reames via llvm-commits]

https://github.com/preames requested changes to this pull request.

Sometimes great minds think alike.  I literally noticed this case last night, and was going to write up the patch this morning.  :)

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