Split the surface UnityPerMaterial CB into three (common, simple lighting and PBS).

ScriptableRenderPipeline/LightweightPipeline/LWRP/ShaderLibrary/InputSurface.hlsl

 #define SAMPLE_METALLICSPECULAR(uv) SAMPLE_TEXTURE2D(_MetallicGlossMap, sampler_MetallicGlossMap, uv)
 #endif
 
-CBUFFER_START(UnityPerMaterial)
+CBUFFER_START(UnityPerMaterial_Common)
+CBUFFER_END
+
+CBUFFER_START(UnityPerMaterial_Simple)
+half _Shininess;
+CBUFFER_END
+
+CBUFFER_START(UnityPerMaterial_PBS)
-half _Shininess;
 CBUFFER_END
 
 TEXTURE2D(_MainTex);            SAMPLER(sampler_MainTex);