[InstCombine] GEPOperator::accumulateConstantOffset does not support scalable vectors

Avoid transforming:

 %0 = bitcast i8* %base to <vscale x 16 x i8>*
 %1 = getelementptr <vscale x 16 x i8>, <vscale x 16 x i8>* %0, i64 1

into:

 %0 = getelementptr i8, i8* %base, i64 16
 %1 = bitcast i8* %0 to <vscale x 16 x i8>*

Reviewers: efriedma, ctetreau

Reviewed By: efriedma

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76236