[TableGen] Make OpcodeMappings sort comparator strict weak ordering compliant

This did not satisfy equivalence of transitivity. There was an attempt
to fix it in https://reviews.llvm.org/D58687 but it was not fully
correct. Masks might not be equivalent but be equal according to LessThan lambda

Reviewed By: aeubanks, MaskRay

Differential Revision: https://reviews.llvm.org/D157955