[CodeGen][SVE] Replace deprecated calls in getCopyFromPartsVector()

Summary: Replaced getVectorNumElements() with getVectorElementCount(). Added operator overloads for class ElementCount. Fixes warning in several AArch64 unit tests.

Reviewers: sdesmalen, kmclaughlin, dancgr, efriedma, each, andwar, rengolin

Reviewed By: efriedma

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80826