Fix Issue 1234.

Fix Issue 1234.

Ensure that there is always enough bytes between consequtive calls in
unoptimized code to write a call instruction at the return points
without overlapping.

This handles the case where two return points were only four bytes
apart (because the latter call was to a register).

BUG=v8:1234

Review URL: http://codereview.chromium.org/6624091

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7089 ce2b1a6d-e550-0410-aec6-3dcde31c8c00