Fix combined string length computation.

Fix combined string length computation.

For 31-bit smis, we don't need to explicitly check whether the
combined string length exceeds the max supported string length,
since the value must be converted to a smi at some point (i.e.
when it is stored into the string length field of the resulting
string), which will emit an overflow check.

For 32-bit smis, we insert an explicit check that the combined
string length does not exceed String::kMaxLength.

This also enables to get rid of the JoinContinuation() usage in
BuildUncheckedStringAdd().

BUG=v8:2990
LOG=n
R=mvstanton@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=18005

[Benedikt Meurer, 2013-11-22 08:17:43]

Hey Michael,
Here's a nice one for you. Finally found a solution to the string length
overflow problem. PTAL :-)
-- Benedikt

[mvstanton, 2013-11-22 12:01:45]

very nice! lgtm.

[Benedikt Meurer, 2013-11-22 12:03:07]

Committed patchset #2 manually as r18005 (presubmit successful).