MIPS: With >= 64 non-string instance types, I removed an optimization that allowed us to detect int…

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=

[mvstanton, 2013-07-19 14:26:22]

thanks for the port! lgtm.

[palfia, 2013-07-19 14:57:12]

Committed as r15783.