Use constant 0 instead of constant null when clearing map words during allocation folding.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/112673002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18356 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Improve ==/=== comparison in Hydrogen for monomorphic maps

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/108083004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18357 ce2b1a6d-e550-0410-aec6-3dcde31c8c00