[AArch64][GlobalISel] Legalize + select v2p0 -> v264 G_PTRTOINT

1) Just mark this case as legal because it can just be a copy.

2) Ensure the copy in the existing code actually gets selected. Without doing
this, we'll crash because the destination won't have a register class.

This fell back 35 times in a build of clang with GISel for AArch64.

Differential Revision: https://reviews.llvm.org/D108610