[builtins] Remove STRING_ADD_LEFT and STRING_ADD_RIGHT builtins.

[builtins] Remove STRING_ADD_LEFT and STRING_ADD_RIGHT builtins.

These builtins present an optimization for the general addition case,
where one side is already known to be a string. Unfortunately this
optimization is wrong in the presence of @@toPrimitive (there are some
ideas how to implement a similar optimization using the prototype
backpointer mechanism that jkummerow@ introduced earlier). So this
also removes the broken %_IsStringWrapperSafeForDefaultValueOf, which is
the key part of the optimization mentioned above.

R=mstarzinger@chromium.org
BUG=v8:4307
LOG=n

Committed: https://crrev.com/d90a404747bd065d2fe653704222b1e6638ef9e6
Cr-Commit-Position: refs/heads/master@{#30707}

[Benedikt Meurer, 2015-09-14 08:00:56]

Hey Michi,

Here's another -770 lines of weird code.
Please take a look.

Thanks,
Benedikt

[Michael Starzinger, 2015-09-14 08:34:43]

LGTM. Love it. Please more of that. As discussed offline, date-format-xparb
tanked.

[Benedikt Meurer, 2015-09-14 08:39:02]

On 2015/09/14 08:34:43, Michael Starzinger wrote:
> LGTM. Love it. Please more of that. As discussed offline, date-format-xparb
> tanked.

Yep, but as discussed, we'll land the optimization separate from the actual
fixification.

[Benedikt Meurer, 2015-09-14 08:39:05]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2015-09-14 08:39:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1336273002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1336273002/1

[commit-bot: I haz the power, 2015-09-14 08:40:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-14 08:41:49]

Patchset 1 (id:??) landed as
https://crrev.com/d90a404747bd065d2fe653704222b1e6638ef9e6
Cr-Commit-Position: refs/heads/master@{#30707}