This fixes potential issues that would appear when trying to insert a breakpoint
from JavaScript.
Without this, computing the mapping of addresses between versions of the code with
and without debug slots could be off by an instruction, trigerring nasty bugs.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/139983007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19350 ce2b1a6d-e550-0410-aec6-3dcde31c8c00