[MIPS] Address instruction selection failure for abs.[sd]

Previously, the choice between the instruction selection of ISD::FABS was
decided at the point of setting the MIPS target lowering operation choice
either `Custom` lowering or `Legal`. This lead to instruction selection
failures as functions could be marked as having no NaNs.

Changing the lowering to always be `Custom` and directly handling the
the cases where MIPS selects the instructions for ISD::FABS resolves
this crash.

Thanks to kray for reporting the issue and to Simon Atanasyan for producing
the reduced test case.

This resolves PR/53722.

Differential Revision: https://reviews.llvm.org/D124651