Next bunch of fixes for check elimination.

Next bunch of fixes for check elimination.

- Canonicalize HCheckMapValue with constant map to
  HCheckMaps, and get rid of the special treatment
  during check elimination.
- Track only stable object maps for HConstants and
  add CHECK()s to verify state during code generation.

R=svenpanne@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=21154

[Benedikt Meurer, 2014-05-06 06:26:02]

PTAL

[Sven Panne, 2014-05-06 06:34:11]

lgtm

[Benedikt Meurer, 2014-05-06 07:05:32]

Committed patchset #1 manually as r21154 (presubmit successful).