AMDGPU: Pattern match fract instructions in AMDGPUCodeGenPrepare

This will allow eliminating the intrinsic uses in the device
libraries, which will remove a subtarget dependency on the f16
version of the intrinsic.

We previously had some wrong patterns for this under unsafe math
which I've removed.

Do it in IR partially to take advantage of the much better isKnownNeverNaN
handling, and partially out of laziness to avoid repeating this in the DAG
and GlobalISel path. Plus I think this should be done much earlier. Ideally
this would be in InstCombine, but you can't introduce target intrinsics
from a generic instruction rooted pattern.