Improve handling of property loads on the proto chain.

Improve handling of property loads on the proto chain.

Previously Crankshaft emitted a generic load for these, now we emit a load of a
named field, guarded by a proto chain check.

LCheckPrototypeMaps now returns the holder, which is for free, because it
already had to check its map as the last step, anyway. This is in sync with what
StubCompiler::CheckPrototype does.

Committed: https://code.google.com/p/v8/source/detail?r=12847

[Sven Panne, 2012-10-31 10:13:41]

Shiny new version of this CL, now with improved holder handling! :-P

[Jakob Kummerow, 2012-10-31 13:44:28]

LGTM.

I'm not too happy with the fact that "{H,L}CheckPrototypeMaps" are now doing
more than just checking something, but I don't have a good proposal for a better
name -- HCheckPrototypeMapsAndReturnHolder would be a bit clunky.