[AArch64][GlobalISel] Fold constants into G_GLOBAL_VALUE

This is pretty much just ports `performGlobalAddressCombine` from
AArch64ISelLowering. (AArch64 doesn't use the generic DAG combine for this.)

This adds a pre-legalize combine which looks for this pattern:

```
  %g = G_GLOBAL_VALUE @x
  %ptr1 = G_PTR_ADD %g, cst1
  %ptr2 = G_PTR_ADD %g, cst2
  ...
  %ptrN = G_PTR_ADD %g, cstN
```

And then, if possible, transforms it like so:

```
  %g = G_GLOBAL_VALUE @x
  %offset_g = G_PTR_ADD %g, -min(cst)
  %ptr1 = G_PTR_ADD %offset_g, cst1
  %ptr2 = G_PTR_ADD %offset_g, cst2
  ...
  %ptrN = G_PTR_ADD %offset_g, cstN
```

Where min(cst) is the smallest out of the G_PTR_ADD constants.

This means we should save at least one G_PTR_ADD.

This also updates code in the legalizer + selector which assumes that
G_GLOBAL_VALUE will never have an offset and adds/updates relevant tests.

Differential Revision: https://reviews.llvm.org/D96624