UPDATE: This does not actually work in its current form! Use nonuniformEXT on available drivers (AMD at the time of writing). GLSL cross-platform non-uniform resource indexing hack (I did not make this)

NonUniformHack.glsl

#extension GL_ARB_shader_ballot : require
uint NonUniform(uint i)
{
  for (;;)
  {
    uint cur = readFirstInvocationARB(i);
    if (cur == i)
    {
      return cur;
    }
  }
}