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

[Amara Emerson via llvm-commits]

https://github.com/aemerson approved this pull request.

I don't have strong objections to it as it's internal to RISC-V.

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