https://reviews.llvm.org/D31536 didn't really solve the problem it was trying to solve; it got rid of the assertion failure, but we were still scheduling the DAG incorrectly (mixing together instructions from different calls), leading to a MachineVerifier failure.
In order to schedule the DAG correctly, we have to make sure we don't schedule a node which should be blocked by an interference. Fix ScheduleDAGRRList::PickNodeToScheduleBottomUp so it doesn't pick a node like that.
The added call to FindAvailableNode() is the key change here; this makes sure we don't try to schedule a call while we're in the middle of scheduling a different call. I'm not sure this is the right approach; in particular, I'm not sure how to prove we don't end up with an infinite loop of repeatedly backtracking.
This also reverts the code change from D31536. It doesn't do anything useful: we should never schedule an ADJCALLSTACKDOWN unless we've already scheduled the corresponding ADJCALLSTACKUP.