[CGP][AArch64] Rebase the common base offset for better ISel

When all the large const offsets masked with the same value from bit-12 to bit-23.
Fold
  add     x8, x0, #2031, lsl #12
  add     x8, x8, #960
  ldr     x9, [x8, x8]
  ldr     x8, [x8, #2056]

into
  add     x8, x0, #2031, lsl #12
  ldr     x9, [x8, #960]
  ldr     x8, [x8, #3016]