Consistently use HStringAdd instead of HCallStub with CodeStub::StringAdd.

Consistently use HStringAdd instead of HCallStub with CodeStub::StringAdd.

Previously there were two ways to actually use the StringAddStub
from Hydrogen:

- Either using HStringAdd (which implied NO_STRING_CHECK_IN_STUB
  and and does the argument handling internally),
- or using HCallStub with CodeStub::StringAdd (which implied
  NO_STRING_ADD_FLAGS and expected the arguments to be on the
  stack already).

R=mvstanton@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=15771

[Benedikt Meurer, 2013-07-19 06:55:43]

Hey Michael,
This is the HStringAdd cleanup we talked about offline the other day.
-- Benedikt

[mvstanton, 2013-07-19 09:40:55]

Looks good to me, esp. since you want to move in the direction that the string
add stub won't need to push arguments on the stack, and may even inline.

lgtm.

[Benedikt Meurer, 2013-07-19 09:42:23]

Committed patchset #1 manually as r15771 (presubmit successful).