MIPS: Fix for Remove unsafe EmitLoadRegister usage in AddI/SubI for constant right operand.

MIPS: Fix for Remove unsafe EmitLoadRegister usage in AddI/SubI for constant right operand.

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

Committed: https://crrev.com/879053a59c426e84f355b9ee03dc3b4e6f9ccc1f
Cr-Commit-Position: refs/heads/master@{#29034}

[akos.palfi.imgtec, 2015-06-16 00:37:11]

akos.palfi@imgtec.com changed reviewers:
+ akos.palfi@imgtec.com

[akos.palfi.imgtec, 2015-06-16 00:37:12]

Lgtm

[paul.l..., 2015-06-16 02:06:39]

paul.lind@imgtec.com changed reviewers:
+ balazs.kilvady@imgtec.com, dusan.milosavljevic@imgtec.com,
mstarzinger@chromium.org, rossberg@chromium.org

[paul.l..., 2015-06-16 02:06:56]

The CQ bit was checked by paul.lind@imgtec.com

[paul.l..., 2015-06-16 02:06:57]

The patchset sent to the CQ was uploaded after l-g-t-m from
akos.palfi@imgtec.com
Link to the patchset: https://codereview.chromium.org/1187063002/#ps20001
(title: "Specify scratch reg to Overflow macros, to avoid 'at' reg conflict.")

[commit-bot: I haz the power, 2015-06-16 02:07:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1187063002/20001

[commit-bot: I haz the power, 2015-06-16 02:29:36]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-16 02:29:57]

Patchset 2 (id:??) landed as
https://crrev.com/879053a59c426e84f355b9ee03dc3b4e6f9ccc1f
Cr-Commit-Position: refs/heads/master@{#29034}