Generated code ended up having two conditional jump statements in a
row. Also introduce JumpIfJSArrayHasAllocationMemento which handles
most cases more simply.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/26841009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17220 ce2b1a6d-e550-0410-aec6-3dcde31c8c00