[llvm] [RISCV][GISel] Do libcall for G_FPTOSI, G_FPTOUI when no D or F support (PR #94613)

[Gábor Spaits via llvm-commits]

spaits wrote:

@topperc I have removed the `legalFor` call for `G_UNMERGE_VALUES`. Also I have added the tests as you have suggested. Now they are using the correct liveins. Now there is no problem with the legality of `G_UNMERGE_VALUES` for libcalls.  Thank you for your help and review.

Could you please re-review this PR?

https://github.com/llvm/llvm-project/pull/94613