[InstSimplify][SVE] Fix SimplifyGEPInst for scalable vector.

Summary:
Skip folds that rely on DataLayout::getTypeAllocSize(). For scalable
vector, only minimal type alloc size is known at compile-time.

Reviewers: sdesmalen, efriedma, spatel, apazos

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75892