Fix a missed addiu->Addu, and a bad default use of 'at' register in https://codereview.chromium.org/1185143002/

TEST=test/mjsunit/regress/regress-500176
BUG=chromium:500176
LOG=N

Review URL: https://codereview.chromium.org/1187063002

Cr-Commit-Position: refs/heads/master@{#29034}