[AArch64][SVE] Remove nxv1f32 and nxv1f64 as legal types

Summary: Also cleans up ZPR register class definition.

Reviewers: sdesmalen, cameron.mcinally, efriedma

Reviewed By: efriedma

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl,
llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71351