[SVE] Eliminate bad VectorType::getNumElements() calls from ConstantFold

Summary:
Assume all usages of this function are explicitly fixed-width operations
and cast to FixedVectorType

Reviewers: efriedma, sdesmalen, c-rhodes, majnemer, dblaikie

Reviewed By: sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80262