[turbofan] Skip write barriers when storing smi.

[turbofan] Skip write barriers when storing smi.

On 64-bit targets, we can skip the write barrier for Store nodes if the
input is ChangeInt32ToTagged, because the value being stored is
definitely represented as a smi then.

R=jarin@chromium.org

Committed: https://crrev.com/155278d6f26f97631c3cf42caaf73927d007bf47
Cr-Commit-Position: refs/heads/master@{#26934}

[Benedikt Meurer, 2015-03-02 06:35:16]

PTAL

[Jarin, 2015-03-02 08:26:45]

lgtm

[Benedikt Meurer, 2015-03-02 09:09:16]

Committed patchset #3 (id:40001) manually as
155278d6f26f97631c3cf42caaf73927d007bf47 (presubmit successful).

[commit-bot: I haz the power, 2015-03-02 09:09:17]

Patchset 3 (id:??) landed as
https://crrev.com/155278d6f26f97631c3cf42caaf73927d007bf47
Cr-Commit-Position: refs/heads/master@{#26934}