[llvm] [GlobalIsel][AArch64] legalize ptr add (PR #89218)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

The new error after  `moreElementsToNextPow2`  is:
```
unable to legalize instruction: %res:_(<3 x p0>) = G_PTR_ADD %ptr:_, %add:_(<3 x s64>) (in function: test_ptr_add_vec_3xp0)
```

https://github.com/llvm/llvm-project/pull/89218