Fixed GI in unlit shader. Removed SampleGI function overloads as it's error prone due to implicit truncation.

ScriptableRenderPipeline/LightweightPipeline/LWRP/ShaderLibrary/Lighting.hlsl

 // If lightmap: sampleData.xy = lightmapUV
 // If probe: sampleData.xyz = L2 SH terms
 #ifdef LIGHTMAP_ON
-#define SAMPLE_GI(lmName, shName, normalWSName) SampleGI(lmName, normalWSName)
-half3 SampleGI(float2 sampleData, half3 normalWS)
-{
-    return SampleLightmap(sampleData, normalWS);
-}
+#define SAMPLE_GI(lmName, shName, normalWSName) SampleLightmap(lmName, normalWSName)
-#define SAMPLE_GI(lmName, shName, normalWSName) SampleGI(shName, normalWSName)
-half3 SampleGI(half3 sampleData, half3 normalWS)
-{
-    // If lightmap is not enabled we sample GI from SH
-    return SampleSHPixel(sampleData, normalWS);
-}
+#define SAMPLE_GI(lmName, shName, normalWSName) SampleSHPixel(shName, normalWSName)
 #endif
 
 half3 GlossyEnvironmentReflection(half3 reflectVector, half perceptualRoughness, half occlusion)

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightStandardUnlit.shader

             {
                 float3 uv0AndFogCoord           : TEXCOORD0; // xy: uv0, z: fogCoord
 #if _SAMPLE_GI
-                float4 lightmapOrVertexSH       : TEXCOORD1;
+                DECLARE_LIGHTMAP_OR_SH(lightmapUV, vertexSH, 1);
                 half3 normal                    : TEXCOORD2;
     #if _NORMALMAP
                 half3 tangent                   : TEXCOORD3;
 
 #if _SAMPLE_GI
                 OUTPUT_NORMAL(v, o);
-                OUTPUT_LIGHTMAP_UV(v.lightmapUV, unity_LightmapST, o.lightmapOrVertexSH.xy);
-                OUTPUT_SH(o.normal, o.lightmapOrVertexSH);
+                OUTPUT_LIGHTMAP_UV(v.lightmapUV, unity_LightmapST, o.lightmapUV);
+                OUTPUT_SH(o.normal, o.vertexSH);
 #endif
                 return o;
             }
     #else
                 half3 normalWS = normalize(IN.normal);
     #endif
-                color *= SampleGI(IN.lightmapOrVertexSH, normalWS);
+                color += SAMPLE_GI(IN.lightmapUV, IN.vertexSH, normalWS);
 #endif
                 ApplyFog(color, IN.uv0AndFogCoord.z);