[SVE] Remove invalid usage of getNumElements in Instructions

Summary:
Remove invalid usage of VectorType::getNumElements in
ShuffleVectorInst::isValidOperands identified by test case
llvm::Analysis/ConstantFolding/vscale-shufflevector.ll. The tested
conditions hold for both fixed width and scalable vectors; use
getElementCount().

Reviewers: efriedma, sdesmalen, c-rhodes, spatel

Reviewed By: sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79212