AMDGPU: Implement llvm.set.rounding (#88587)

Use a shift of a magic constant and some offseting to convert from
flt_rounds values.

I don't know why the enum defines Dynamic = 7. The standard suggests -1
is the cannot determine value. If we could start the extended values at
4 we wouldn't need the extra compare sub and select.

https://reviews.llvm.org/D153257