Vertex shaders default to highp precision. Fragment shaders don't have a default precision and you must specify highp/mediump/lowp either via the precision statement (precision highp float) or declare each variable in the shader with the required precision (mediump vec4 sum).
Microsoft published some documentation on new wave level functions in SM6.0:
https://msdn.microsoft.com/en-us/library/windows/desktop/mt733232(v=vs.85).aspx
