MIPS: The check for internalized strings relied on the fact that we had less than 64 distinct InstanceTypes.

Port r15358 (3af77be)

Original commit message:
The check for internalized strings relied on the fact that we had less
than 64 distinct InstanceTypes. We are hitting that boundary, so this
check needs to be more comprehensive. In fact, two bits need to be
tested: verify that kNotStringTag isn't set, and that kInternalizedTag
is set.

BUG=

Review URL: https://codereview.chromium.org/18083004
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15368 ce2b1a6d-e550-0410-aec6-3dcde31c8c00