[ARM] Reverse PostRASched subtarget feature logic

Replace the UsePostRAScheduler SubtargetFeature with
DisablePostRAScheduler, which is then used by Swift and Cyclone.
This patch maintains enabling PostRA scheduling for other Thumb2
capable cores and/or for functions which are being compiled in Arm
mode.

Differential Revision: https://reviews.llvm.org/D37055

llvm-svn: 312226