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
938ed8ae99