Register t3 is not allocatable, so register a7 is used instead.

Change-Id: Ic198f038a10b4c891b417656d639477aedf6b703
Reviewed-on: https://chromium-review.googlesource.com/891823Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#50979}