[llvm] [RISCV] Legalize shuffle index after splitting two argument shuffles (PR #79330)

[Philip Reames via llvm-commits]

preames wrote:

I went ahead and added test coverage for both the original code and the new case.  In the process, I discovered that one my earlier refactorings actually broke this logic - I think my mental state was assuming the prior version of this patch despite it not having landed - and I landed a separate change to get us back to a working baseline.  

Pushed a reworked change, please take a look at the description from scratch.

https://github.com/llvm/llvm-project/pull/79330