[ConstantFold][SVE] Skip scalable vectors in ConstantFoldInsertElementInstruction.

Summary:
Should not constant fold insertelement instruction for scalable vector type.

Reviewers: huntergr, sdesmalen, spatel, levedev.ri, apazos, efriedma, willlovett

Reviewed By: efriedma, spatel

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70985