AArch64: Do not test for CPUs, use SubtargetFeatures

Testing for specific CPUs has a number of problems, better use subtarget
features:
- When some tweak is added for a specific CPU it is often desirable for
  the next version of that CPU as well, yet we often forget to add it.
- It is hard to keep track of checks scattered around the target code;
  Declaring all target specifics together with the CPU in the tablegen
  file is a clear representation.
- Subtarget features can be tweaked from the command line.

To discourage people from using CPU checks in the future I removed the
isCortexXX(), isCyclone(), ... functions. I added an getProcFamily()
function for exceptional circumstances but made it clear in the comment
that usage is discouraged.

Reformat feature list in AArch64.td to have 1 feature per line in
alphabetical order to simplify merging and sorting for out of tree
tweaks.

No functional change intended.

Differential Revision: http://reviews.llvm.org/D20762

llvm-svn: 271555