Refactor AddPair and SubPair Macro-assembler instructions to
prevent register overwriting, refactor all the places where
these instructions are used. Also, implement MulPair instruction.

Change-Id: I3f8f9d5fe6fa5bf25df3446614ac311cf886b6ac
Reviewed-on: https://chromium-review.googlesource.com/1013571Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Sreten Kovacevic <sreten.kovacevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#52642}