MIPS: Fix bug in StringConstructCode builtin.

MIPS: Fix bug in StringConstructCode builtin.

BUG=

TEST=webkit/dfg-to-string-bad-toString, webkit/dfg-to-string-bad-valueOf,
webkit/dfg-to-string-int-or-string, webkit/dfg-to-string-side-effect,
webkit/dfg-to-string-bad-toString, webkit/dfg-to-string-bad-valueOf,
webkit/dfg-to-string-toString-in-string, webkit/dfg-to-string-int,
webkit/dfg-to-string-valueOf-in-string

Committed: https://code.google.com/p/v8/source/detail?r=15489

[kisg, 2013-07-04 14:03:35]

LGTM

[Yang, 2013-07-04 15:04:23]

On 2013/07/04 14:03:35, kisg wrote:
> LGTM

LGTM.

[palfia, 2013-07-04 15:19:38]

Committed as r15489.