[NFC] Declare the member data of class PostGenericScheduler as "protected" instead of "private"

Some target might try to subclass the PostGenericScheduler to custom the scheduling strategy.
We need to declare the member data of PostGenericScheduler as "protected", which acts the same as "GenericScheduler".

Differential Revision: https://reviews.llvm.org/D58949

llvm-svn: 355475