[FastISel][AArch64] Move over to target-dependent instruction selection only.

This change moves FastISel for AArch64 to target-dependent instruction selection
only. This change replicates the existing target-independent behavior, therefore
there are no changes to the unit tests or new tests.

Future changes will take advantage of this change and update functionality
and unit tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216955 91177308-0d34-0410-b5e6-96231b3b80d8