MIPS: Improve polymorphic loads on single slots.

MIPS: Improve polymorphic loads on single slots.

Port r11132 (91bdad6108).

Original commit message:

If all property lookups for a polymorphic load actually result in the
same field index under all maps, we can actually emit a monomorphic load
that is guarded by a map check that verifies that the actual map is in
the set of handled maps. This also allows GVN to get rid of redundant
such map checks.

BUG=
TEST=

Committed: https://code.google.com/p/v8/source/detail?r=11135

[Michael Starzinger, 2012-03-26 09:42:37]

LGTM. Landed.