AArch64: Implement InexpensiveConstant methods.

AArch64: Implement InexpensiveConstant methods.

Implement IsInexpensiveConstant and friends for A64.
Also extending the methods to take the opcode with respect to which
the constant is inexpensive. Additionally, logical operations (i.e.
and, or, xor) can now handle the immediates 0 and ~0 (which are not
logical immediates).

Change-Id: I46ce1287703765c5ab54983d13c1b3a1f5838622