[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
> 
> These MIR tests don't work:
> 
> ```
> ---
> name:            frem_v2f32
> body:             |
>   bb.0.entry:
>     ; unable to legalize instruction: %3:_(s32), %4:_(s32) = G_UNMERGE_VALUES %0:_(<2 x s32>)

If you use -global-isel-abort=2, you can check the partially legalized output 

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