[GlobalISel][Mips] Sync G_UADDE and G_USUBE legalization with LegalizeDAG.

This modifies the G_UADDE legalizaton to a version that looks shorter
on Mips and RISC-V when feeding the equivalent IR to SelectionDAG.
This also removes the boolean select from G_USUBE.

Comments taken from LegalizeDAG and tweaked.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D158232