[turbofan] Avoid some unnecessary write barriers.

[turbofan] Avoid some unnecessary write barriers.

Use the type specified for StoreField and StoreElement to check whether
we are storing a TaggedSigned value, and skip the write barrier for the
write completely in that case.  Also make sure to set that field type
appropriately for JSGlobalObjectSpecialization.

R=jarin@chromium.org
BUG=v8:4470
LOG=n

Committed: https://crrev.com/bdc950551428c826e60e502304f207b32bf65a83
Cr-Commit-Position: refs/heads/master@{#31772}

[Benedikt Meurer, 2015-11-04 08:44:58]

Hey Jaro,

Here's a simple fix to avoid some stupid write barriers.
Please take a look.

Thanks,
Benedikt

[Jarin, 2015-11-04 08:58:20]

lgtm

[Benedikt Meurer, 2015-11-04 09:00:11]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-11-04 09:00:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1410743009/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1410743009/1

[commit-bot: I haz the power, 2015-11-04 09:09:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-04 09:10:15]

Patchset 1 (id:??) landed as
https://crrev.com/bdc950551428c826e60e502304f207b32bf65a83
Cr-Commit-Position: refs/heads/master@{#31772}