[SVE] Add specialized getters to derived vector types

Summary:
Add versions of the specialized getters in base VectorType to the
derived vector types that return derived pointers. This reduces the
amount of casting you need to do when working with derived vectors

Reviewers: efriedma, david-arm, fpetrogalli, craig.topper

Reviewed By: david-arm

Subscribers: tschuett, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80026