The former default was true. The new default is false and the default
implementation is UNREACHABLE so it asserts in debug builds.  The function
is overridden in all concrete instruction classes that might have the flag
kUseGVN set.

Review URL: http://codereview.chromium.org/6255013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6493 ce2b1a6d-e550-0410-aec6-3dcde31c8c00