[llvm] [AArch64] merge index address with large offset into base address (PR #72187)

[via llvm-commits]

vfdff wrote:

> It might be worth doing this as a (late) AArch64ISelLowering DAG combine so that the add gets shared between multiple load/store instructions.
* Yes, the current solution does not implement the reuse of add, eg:
```
define i64 @LdOffset_i64_multi_offset(ptr %a) {
; CHECK-LABEL: LdOffset_i64_multi_offset:
; CHECK:       // %bb.0: // %entry
; CHECK-NEXT:    add x8, x0, #2031, lsl #12 // =8318976
; CHECK-NEXT:    add x9, x0, #2031, lsl #12 // =8318976
; CHECK-NEXT:    add x8, x8, #960
; CHECK-NEXT:    ldr x9, [x9, #960]
; CHECK-NEXT:    ldr x8, [x8, #2056]
; CHECK-NEXT:    add x0, x8, x9
; CHECK-NEXT:    ret
entry:
  %arrayidx = getelementptr inbounds i64, ptr %a, i64 1039992
  %0 = load i64, ptr %arrayidx, align 8
  %arrayidx1 = getelementptr inbounds i64, ptr %a, i64 1040249
  %1 = load i64, ptr %arrayidx1, align 8
  %add = add nsw i64 %1, %0
  ret i64 %add
}
```
* In that case, I think it's reasonable to go back to my previous plan? （I'll add more test cases if it's right direction).
   I don't prefer to `DAG combine` because it seems we need similar code to address `load` and `store` separately
  https://github.com/llvm/llvm-project/commit/bff3b81d921a7c69fd8a50dcbf7cd44776c73d19

https://github.com/llvm/llvm-project/pull/72187