[SVE] Fix invalid usage of VectorType::getNumElements() in InstCombine

Summary:
Make foldVectorBinop return null if the instruction type is a scalable
vector. It is unclear what, if any, of this function works with scalable
vectors.

Identified by test LLVM.Transforms/InstCombine::nsw.ll

Reviewers: efriedma, david-arm, fpetrogalli, spatel

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79196