CodeGen: Move check of EnablePostRAScheduler to avoid disabling antidependency breaker

Previously, specifying -post-RA-scheduler=true had the side effect of
disabling the antidependency breaker, yielding different behavior than
if the post-RA-scheduler was enabled via the scheduling model.

Differential Revision: http://reviews.llvm.org/D20186

llvm-svn: 270077