Optimize AArch64 code generator

- use cbz/cbnz for comparison with zero
- use xzr/wzr instead of #0 in mov/cmp
- use "subs" imstead of "sub x1,...; cmp x1, xzr"
- avoid loading immediate value to register when it may be direclu used
  as a source immendiate operand in ldr*/str*/add*/sub*/cmp

Author: dstogov