[ConstantFold][SVE] Fix constand fold for vector call.

Summary:
Do not iterate on scalable vectors.

Reviewers: sdesmalen, efriedma, apazos, huntergr, willlovett

Reviewed By: sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74419