[SVE] Add specialized overloads of VectorType::get

Summary:
Add overloads of get to FixedVectorType and ScalableVectorType that
mirror VectorType::get(Type *Ty, VectorType *VTy) that take and return
derived vector types

Reviewers: efriedma, c-rhodes, david-arm, fpetrogalli

Reviewed By: david-arm

Subscribers: tschuett, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78841