Experimental: push the new code generator infrastructure into the...

Experimental: push the new code generator infrastructure into the
count operations (pre- and post- increment and decrement).

For frame-allocated slots in registers, we can overwrite them.  This
is a simple way to reduce register pressure by not consuming a
register to back the frame element that will be written before being
read again.  It also ensures that frame-allocated slots that are the
target of count operations do not migrate to a different register,
which would make merging generate silly moves.

This change allows INVALID frame elements to appear in expected
frames.  It is always safe to merge to invalid elements.  Currently we
require both source and target elements to be invalid, but that could
be relaxed.

Committed: http://code.google.com/p/v8/source/detail?r=1017

[William Hesse, 2008-12-22 15:33:07]

LGTM