Change is performance neutral but generates smaller code and encapsulates double alignment in the macro-assembler rather than at the allocation site.

Review URL: https://codereview.chromium.org/11684005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13284 ce2b1a6d-e550-0410-aec6-3dcde31c8c00