Simple support for const variables in Crankshaft.

Simple support for const variables in Crankshaft.

The approach is to handle the common case in the optimizing
compiler and to bailout for the rare corner cases.

This is done by initializing all local const-variables with
the hole value and disallowing any use of the hole value statically.
Review URL: http://codereview.chromium.org/6026006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8104 ce2b1a6d-e550-0410-aec6-3dcde31c8c00