[MachineScheduler] NFC refactoring

This patch makes tryCandidate() virtual and some utility functions like
tryLess(), tryGreater(), ... externally available (used to be static).

This makes it possible for a target to derive a new MachineSchedStrategy from
GenericScheduler and reuse most parts.

It was necessary to wrap functions with the same names in
AMDGPU/SIMachineScheduler in a local namespace.

Review: Andy Trick, Florian Hahn
https://reviews.llvm.org/D43329

llvm-svn: 329884