We may encounter an invalid frame after generating code
for the loop body in case the loop body ends in an unconditional
return. Before setting the type information for the loop variable
we need to check for a valid frame.

Review URL: http://codereview.chromium.org/1106002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4182 ce2b1a6d-e550-0410-aec6-3dcde31c8c00