Save one instruction in allocating new space by using an immediate add if
possible to calculate the new top of heap.

BUG=

Review URL: https://chromiumcodereview.appspot.com/11091068
Patch from Anthony Berent <aberent@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12718 ce2b1a6d-e550-0410-aec6-3dcde31c8c00