[ValueTracking][SVE] Fix getOffsetFromIndex for scalable vector.

Summary:
Return None if GEP index type is scalable vector. Size of scalable vectors
are multiplied by a runtime constant.

Avoid transforming:
  %a = bitcast i8* %p to <vscale x 16 x i8>*
  %tmp0 = getelementptr <vscale x 16 x i8>, <vscale x 16 x i8>* %a, i64 0
  store <vscale x 16 x i8> zeroinitializer, <vscale x 16 x i8>* %tmp0
  %tmp1 = getelementptr <vscale x 16 x i8>, <vscale x 16 x i8>* %a, i64 1
  store <vscale x 16 x i8> zeroinitializer, <vscale x 16 x i8>* %tmp1

into:
  %a = bitcast i8* %p to <vscale x 16 x i8>*
  %tmp0 = getelementptr <vscale x 16 x i8>, <vscale x 16 x i8>* %a, i64 0
  %1 = bitcast <vscale x 16 x i8>* %tmp0 to i8*
  call void @llvm.memset.p0i8.i64(i8* align 16 %1, i8 0, i64 32, i1 false)

Reviewers: sdesmalen, efriedma, apazos, reames

Reviewed By: sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76464