[AArch64][GlobalISel] Improve legalization of G_PTR_ADD (#91763)

The testing we have for vector ptradd was a bit lacking. In adding tests
this patch found a couple of issues mostly with the way v3 vectors of
ptrs were sometimes legalized via i64, and with non-i64 additions. It
does not attempt to fix the issue with mergevalues from returning vector
ptrs.