Reland "Fix phis for non-sse2 double values"

Remove VerifyX87StackDepth from non-debug code.

BUG=
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/23621038

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16682 ce2b1a6d-e550-0410-aec6-3dcde31c8c00