[AArch64][GlobalISel] Mark v2s64 -> v2p0 G_INTTOPTR as legal

Allow

```
%x:_<2 x p0> = G_INTTOPTR %y:_<2 x s64>
```

This shows up when building clang for AArch64 with GlobalISel.

Also show that we can select it.

This should match SDAG's behaviour: https://godbolt.org/z/33oqYoaYv

Differential Revision: https://reviews.llvm.org/D105944