[llvm] [GISel][RISCV] Legalize `G_FREM` to use fmod (PR #93063)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > It should be possible to test the scalarize case
> 
> I tried the following 4 variants, but no one works :(

This is the case where MIR tests are useful  because you don't have to bring up all the other parts 

https://github.com/llvm/llvm-project/pull/93063