At is used in Macro Assembler, so we need other registers to hold temporary values.

Change-Id: Iffeddba7b3319666a605eea62ecc3cd01b065ad7
Reviewed-on: https://chromium-review.googlesource.com/1013978Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Sreten Kovacevic <sreten.kovacevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#52623}