This patch fixes register overwrite in ShrPair, ShlPair and SarPair
instructions. Additionally, we rename kLithiumScratch register
register since lithium is not present anymore.

Change-Id: I65861c4f27d2161bcf49cf02ca8987eb82c997ea
Reviewed-on: https://chromium-review.googlesource.com/1012110Reviewed-by: Sreten Kovacevic <sreten.kovacevic@mips.com>
Commit-Queue: Sreten Kovacevic <sreten.kovacevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#52593}