(sorry for the link which is not working outside the MV network)
This assertion show-up for 1/3 of the tests on ARM after register allocation.
I've already fixed this, but the actual change is in both the patch that dvander is reviewing, and the patch that sstangl is reviewing. Whenever one of them completes their review, that'll get pushed.
It seems that patch for Bug 714949 attachment 585546 [details] [diff] [review] fix this bug.