AArch64: improve 64-bit immediates loads.

Improve the quick backend to load immediates by choosing the best
of the following strategies:

  - use wzr, xzr to load 0 (via mov) or -1 (via mvn),
  - use logical immediates (orr),
  - use one movz/movn optionally followed by one or more movk,
  - use the literal pool.

Change-Id: I8e46e6d9eaf46b717761dd9d60e63ee3f2a5422b