Use std::set instead of std::priority_queue for the RegReductionPriorityQueue.

This removes the existing bottleneck related to the removal of elements from 
the middle of the queue.

Also fixes a subtle bug in ScheduleDAGRRList::CapturePred:
It was updating the state of the SUnit before removing it. As a result, the
comparison operators were working incorrectly and this SUnit could not be removed 
from the queue properly.

Reviewed by Evan and Dan. Approved by Dan.

llvm-svn: 50412