Revert part of 11727 as it sometimes tanked V8 benchmark (raytrace) performance

Revert part of 11727 as it sometimes tanked V8 benchmark (raytrace) performance
for reasons that are not obvious.  Now we make objects into fast-case objects
when they are made prototypes for other objects, but we do not mark objects
that are already fast case with a bit that helps keep them in fast case.
Committed: https://code.google.com/p/v8/source/detail?r=11831

[ulan, 2012-06-15 11:42:18]

LGTM (rubber-stamp).

Could you please add to the description that it is a partial revert?

I checked that it fixed the raytrace regression on my machine.