MIPS: With >= 64 non-string instance types, I removed an optimization that allowed us to detect internalized strings with a single bit test.

Port r15773 (60aa9f0)

Original commit message:
With >= 64 non-string instance types, I removed an optimization that allowed us to detect internalized strings with a single bit test. (https://code.google.com/p/v8/source/detail?r=15358) But that change caused a regression, so here is an improvement:

Put the internalized string types in the lower 64 entries of
INSTANCE_TYPE, and non-internalized string types in the next 64 entries.
This way we can restore the single bit check.

BUG=

Review URL: https://codereview.chromium.org/19723008
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15783 ce2b1a6d-e550-0410-aec6-3dcde31c8c00