[turbofan] Enable typed lowering of string addition.

[turbofan] Enable typed lowering of string addition.

Unfortunately StringAdd is not pure in V8 because we might throw an
exception if the resulting string length is outside the valid bounds, so
there's no point in having a simplified StringAdd operator.

R=jarin@chromium.org

Committed: https://crrev.com/86b30420a844f3da33d9a9d4765b46d0c4554454
Cr-Commit-Position: refs/heads/master@{#28747}

[Benedikt Meurer, 2015-06-02 06:19:39]

Hey Jaro,

Clean up after the decision on string add lowering was made. We have to throw a
RangeError if the strings are too large to concatenate, so no StringAdd
simplified operator unfortunately... Please take a look.

Thanks,
Benedikt

[Jarin, 2015-06-02 08:36:38]

lgtm

[Benedikt Meurer, 2015-06-02 08:49:14]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-06-02 08:49:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1164743002/20001

[commit-bot: I haz the power, 2015-06-02 08:50:51]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-02 08:51:01]

Patchset 2 (id:??) landed as
https://crrev.com/86b30420a844f3da33d9a9d4765b46d0c4554454
Cr-Commit-Position: refs/heads/master@{#28747}