AMDGPU: Avoid instantiating PatFrag with null_frag (#77271)

This makes it possible to pass null_frag to the MAIInst multiclass.

null_frag does not work as you may hope if used as the input to a
PatFrag, which is what happens when it's passed through to *MAIFrag.
Avoid this by checking for null_frag. It might be possible to hack up
tablegen to allow consuming PatFrag inputs.