[SVE][NFC] Use ScalableVectorType in CGBuiltin

Summary: * Upgrade some usages of VectorType to use ScalableVectorType

Reviewers: efriedma, david-arm, fpetrogalli, kmclaughlin

Reviewed By: efriedma

Subscribers: tschuett, rkruppe, psnobl, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78842