MIPS: Make TestJSArrayForAllocationMemento less awkward.

Port r17220 (be968d52)

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

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/27421002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17225 ce2b1a6d-e550-0410-aec6-3dcde31c8c00