[FastISel][AArch64] Add target-dependent instruction selection for Add/Sub.

There is already target-dependent instruction selection support for Adds/Subs to
support compares and the intrinsics with overflow check. This takes advantage of
the existing infrastructure to also support Add/Sub, which allows the folding of
immediates, sign-/zero-extends, and shifts.

This fixes rdar://problem/18207316.