MIPS: Change generated code for substring not to bail out to runtime system.
MIPS: Change generated code for substring not to bail out to runtime system.

Port r10012 (09c6bcba81).

Original commit message:
There is no need to bail out to runtime system when creating sliced strings of external strings since the string content is unimportant.

BUG=
TEST=

Review URL: http://codereview.chromium.org/8585041

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@10027 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh