MIPS64: Megamorphic KeyedLoadIC needs special handling for vector ics.

MIPS64: Megamorphic KeyedLoadIC needs special handling for vector ics.

Port 16843e239d95f5e8eaaa06d2a0a6e371298fe11f

Original commit message:
When --vector-ics is true, we still tail-call to the hand-written
megamorphic KeyedLoadIC (formerly "generic"). Now that this code uses
the megamorphic cache, it needs to deal properly with the vector and
slot registers. Achieve this with a sentinel vectors/slot combo.

BUG=

Committed: https://crrev.com/6019cbf9dfe5893283b9df86613cf1afddc7e344
Cr-Commit-Position: refs/heads/master@{#26384}

[balazs.kilvady, 2015-02-02 16:56:50]

balazs.kilvady@imgtec.com changed reviewers:
+ akos.palfi@imgtec.com, danno@chromium.org, dusan.milosavljevic@imgtec.com,
gergely.kis@imgtec.com, mvstanton@chromium.org, paul.lind@imgtec.com

[dusmil.imgtec, 2015-02-02 16:59:50]

lgtm.

[balazs.kilvady, 2015-02-02 17:19:14]

The CQ bit was checked by balazs.kilvady@imgtec.com

[commit-bot: I haz the power, 2015-02-02 17:20:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/894053002/1

[commit-bot: I haz the power, 2015-02-02 17:50:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-02 17:50:31]

Patchset 1 (id:??) landed as
https://crrev.com/6019cbf9dfe5893283b9df86613cf1afddc7e344
Cr-Commit-Position: refs/heads/master@{#26384}