[AArch64] merge index address with large offset into base address

A case for this transformation, https://gcc.godbolt.org/z/nhYcWq1WE
Fold
  mov     w8, #56952
  movk    w8, #15, lsl #16
  ldrb    w0, [x0, x8]
into
  add     x0, x0, 1036288
  ldrb    w0, [x0, 3704]

Only LDRBBroX is supported for the first time.
Fix https://github.com/llvm/llvm-project/issues/71917