[AMDGPU] Remove HasAtomicFaddInstsGFX90X and HasAtomicFaddInstsGFX940

These compound predicates are not required, since we can use a
combination of setting the SubtargetPredicate (to a subtarget
predicate like isGFX940Plus) and OtherPredicates (to a list of feature
predicates like HasAtomicFaddInsts) instead. NFC.

Differential Revision: https://reviews.llvm.org/D121289