Rename TargetSubtargetInfo::enablePostMachineScheduler() to enablePostRAScheduler()

r213101 changed the behaviour of this method to not only affect the
PostMachineScheduler scheduler but also the PostRAScheduler scheduler,
renaming should make this fact clear. Also document that the preferred
way is to specify this in the scheduling model instead of overriding
this method.

Differential Revision: http://reviews.llvm.org/D10427

llvm-svn: 239659