Allocate the context in the prologue. Two issues had to be solved:
(1) deoptimization needs to handle functions with a local context,
(2) we need a safepoint in the prologue.
(Thanks to Kevin.)

Review URL: http://codereview.chromium.org/6534022

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6903 ce2b1a6d-e550-0410-aec6-3dcde31c8c00