MIPS: Optimize add, sub and shift pair macro instructions.

The CL adds optimizations for AddPair, SubPair, ShlPair, ShrPair and
SarPair macro instructions.

BUG=

Change-Id: I56460624adc0aa8ae135533ef4b99e0ed8360ccb
Reviewed-on: https://chromium-review.googlesource.com/555513Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Commit-Queue: Miran Karić <Miran.Karic@imgtec.com>
Cr-Commit-Position: refs/heads/master@{#46363}