[AArch64][GlobalISel] Clamp <n x p0> vecs when legalizing G_EXTRACT_VECTOR_ELT

This case was missing from G_EXTRACT_VECTOR_ELT. It's the same as for s64.

https://godbolt.org/z/Tnq4acY8z

Differential Revision: https://reviews.llvm.org/D105952