[PATCH] [AArch64] Improve and enable the SeparateConstOffsetFromGEP for AArch64 backend.

[Jingyue Wu]

I mean the issue we spotted previously with the following example:
```
struct S {
  int a[4]; // sizeof(int) = 4
  double b[8]; // sizeof(double) = 8
};

%s = alloca struct %S
%p = getelementptr %s, 0, 1, i ; &s.b[i] = s + 4*sizeof(int) + i*sizeof(double)
```

I thought we agreed that your diff had a correctness issue with this example, and I didn't see any change to fix that issue in your latest diff. Am I missing something?

http://reviews.llvm.org/D5864