pre-RA-sched assertion fix. This bug was exposed by r176037.

rdar:13370002 [pre-RA-sched] assertion: released too many times

I tracked this down to an earlier hack that is no longer applicable
and interfered with normal scheduler logic. With the changes in
r176037, it was causing an instruction to be scheduled multiple times.

I have an external test case that I tried hard to reduce and
failed. I can't even reproduce with llc.