MIPS: Use a special EnumLength field to indicate number of valid enum cache values.

MIPS: Use a special EnumLength field to indicate number of valid enum cache values.

Port r12400 (03ae62de)

Original commit message:
This is preparatory work for sharing Enum Caches.

BUG=
TEST=

Committed: https://code.google.com/p/v8/source/detail?r=12417

[Toon Verwaest, 2012-08-31 09:49:32]

LGTM, landing.