[FastISel][AArch64] Factor out ADDS/SUBS instruction emission and add support for extensions and shift folding.
Factor out the ADDS/SUBS instruction emission code into helper functions and
make the helper functions more clever to support most of the different ADDS/SUBS
instructions the architecture support. This includes better immedediate support,
shift folding, and sign-/zero-extend folding.
This fixes rdar://problem/17913111.