[SVE] Remove VectorType::getBitWidth()

Summary:
* VectorType::getBitWidth() is just an unsafe version of
getPrimitiveSizeInBits() that assumes all vectors are fixed width.

Reviewers: efriedma, sdesmalen, huntergr, craig.topper

Reviewed By: efriedma

Subscribers: tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77833