MIPS: Vector-ICs - speed towards the monomorphic exit as quickly as possible.

MIPS: Vector-ICs - speed towards the monomorphic exit as quickly as possible.

Port 35a67b745d862c5b1f584c2786ebf1369dc4f1ff

Original commit message:
Thanks to some careful assumptions, we can examine the object found at
vector[slot] and trust it's a heap object where the second field is
either a map if it's a WeakCell, or definitely not a map if it's a
Symbol, String or FixedArray. Use this to save a memory read.

BUG=

Committed: https://crrev.com/b45a664f7bc8155d226df7efebd23fb59e5c6162
Cr-Commit-Position: refs/heads/master@{#27757}

[balazs.kilvady, 2015-04-10 11:58:31]

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

[paul.l..., 2015-04-10 14:11:56]

rs-lgtm

[akos.palfi.imgtec, 2015-04-10 14:58:27]

LGTM

[balazs.kilvady, 2015-04-10 15:37:47]

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

[commit-bot: I haz the power, 2015-04-10 15:37:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1053843003/1

[commit-bot: I haz the power, 2015-04-10 18:28:20]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-10 18:28:26]

Patchset 1 (id:??) landed as
https://crrev.com/b45a664f7bc8155d226df7efebd23fb59e5c6162
Cr-Commit-Position: refs/heads/master@{#27757}