[llvm] [RISCV][GISel] Support G_MERGE_VALUES/G_UNMERGE_VALUES with Zfa. (PR #120379)

[Craig Topper via llvm-commits]

topperc wrote:

> If you don't want to implement a PostLegalizerLowering pass like AArch64 then I guess you could do it in `preISelLower()` instead? It's your call though, if you think this is ok for now go ahead. It's fairly easy to fix later if needed.

I think we tried to add a PostLegalizerLowering pass for something vector related in the past  and got some negative feedback.

https://github.com/llvm/llvm-project/pull/120379