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

Review URL: https://codereview.chromium.org/82733003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18005 ce2b1a6d-e550-0410-aec6-3dcde31c8c00