Save one branch for normal heap number un-tagging.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/23496041

Patch from Bangfu Tao <bangfu.tao@samsung.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16856 ce2b1a6d-e550-0410-aec6-3dcde31c8c00