Optimize calls to evals. Most of the time there is no reason to

Optimize calls to evals. Most of the time there is no reason to
perform a context lookup in the runtime system for the 'eval'
function. Instead load the 'eval' function from the global context in
generated code if it is not shadowed.

Will port to other platforms as a separate change.

Review URL: http://codereview.chromium.org/2666001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4794 ce2b1a6d-e550-0410-aec6-3dcde31c8c00