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

[Matt Arsenault via llvm-commits]

arsenm wrote:

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

I still consider a pre selection lowering a hack papering over missing selection patterns or missing legalization 

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