Micro-optimizations to pop() and shift()

R=verwaest@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/247613002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21043 ce2b1a6d-e550-0410-aec6-3dcde31c8c00