[SVE] Generalize size checks in Verifier to use getElementCount

Summary:
Attempts to call getNumElements on scalable vectors identified by test
LLVM.Other::scalable-vectors-core-ir.ll. Since these checks are all
attempting to find if two vectors are the same size, calling
getElementCount will only increase safety.

Reviewers: efriedma, aprantl, reames, kmclaughlin, sdesmalen

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81895