Fixing bugs in promotion of elements transitions (r1175).

Fixing bugs in promotion of elements transitions (r1175).
- Fixed invalid memory access when reading enum-cache from descriptor array with elements transitions but 0 real descriptors.
- Fixed infinite recursion in the intrusive map iterator when visiting elements transitions.
- Properly cached non-fastmode elements transitions.

Committed: https://code.google.com/p/v8/source/detail?r=11841

[Toon Verwaest, 2012-06-18 11:10:38]

PTAL.

Reduced version of http://codereview.chromium.org/10554011 containing only the
critical fixes.

[danno, 2012-06-18 11:12:39]

lgtm