[System Model] [TTI] Update cache and prefetch TTI interfaces

Re-apply 9fdfb045/r365676 with fixes for PPC and Hexagon.  This involved
moving defaults from TargetTransformInfoImplBase to MCSubtargetInfo.

Rework the TTI cache and software prefetching APIs to prepare for the
introduction of a general system model.  Changes include:

- Marking existing interfaces const and/or override as appropriate
- Adding comments
- Adding BasicTTIImpl interfaces that delegate to a subtarget
  implementation
- Moving the default TargetTransformInfoImplBase implementation to a default
  MCSubtarget implementation

Only a handful of targets use these interfaces currently: AArch64, Hexagon, PPC
and SystemZ.  AArch64 already has a custom subtarget implementation, so its
custom TTI implementation is migrated to use the new facilities in BasicTTIImpl
to invoke its custom subtarget implementation.  The custom TTI implementations
continue to exist for the other targets with this change.  They are not moved
over to subtarget-based implementations.

The end goal is to have the default subtarget implementation defer to the system
model defined by the target.  With this change, the default MCSubtargetInfo
implementation essentially returns the defaults TargetTransformInfoImplBase used
to return.  Existing users of TTI defaults will hit the defaults now in
MCSubtargetInfo.  Targets that define their own custom TTI implementations won't
use the BasicTTIImpl implementations that route to the subtarget.

Once system models are in place for the targets that use these interfaces, their
custom TTI implementations can be removed.

Differential Revision: https://reviews.llvm.org/D63614

llvm-svn: 374205