AMDGPU: llvm.amdgcn.writelane is a source of divergence

Summary:
Consider:

  %r = call i32 @llvm.amdgcn.writelane(i32 0, i32 1, i32 2)

This produces a value that is 0 on lane 1, and 2 everywhere else; i.e.,
it is divergent.

Reported-by: Marek Olsak <Marek.Olsak@amd.com>

Reviewers: arsenm, foad, mareko

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74400