Don't generate useless string checks for string adds.

Don't generate useless string checks for string adds.

If we know that one side of a string add is definitely a string
(i.e. if it's a string constant), then we don't need to emit a
string check for the argument.

This adds a new BuildCheckString() method to the graph builder,
which does "the right thing".

TEST=mjsunit/string-add
R=mvstanton@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=17912

[Benedikt Meurer, 2013-11-20 10:15:23]

Hey Michael,
This is an obvious optimization for inline binary operations in crankshaft. PTAL
-- Benedikt

[mvstanton, 2013-11-20 11:51:08]

Be sure to use the method soon :). lgtm.

[Benedikt Meurer, 2013-11-20 11:53:15]

Committed patchset #1 manually as r17912.