Fixed compiler errors in LW.

ScriptableRenderPipeline/LightweightPipeline/LWRP/ShaderLibrary/Lighting.hlsl

 half4 LightweightFragmentLambert(float3 positionWS, half3 normalWS, half3 viewDirectionWS,
     half fogFactor, half3 bakedGI, half3 diffuse, half3 emission, half alpha)
 {
-    half3 lightDirection;
-
     Light mainLight = GetMainLight(positionWS);
     half3 indirectDiffuse = SubtractDirectMainLightFromLightmap(mainLight, normalWS, bakedGI);
     half3 lambert = LightingLambert(mainLight.color, mainLight.direction, normalWS);
     {
         Light light = GetLight(i, positionWS);
         half3 attenuatedLightColor = light.color * light.attenuation;
-        diffuseColor += LightingLambert(attenuatedLightColor, lightDirection, normalWS);
+        diffuseColor += LightingLambert(attenuatedLightColor, light.direction, normalWS);
     }
 #endif
 
 half4 LightweightFragmentBlinnPhong(float3 positionWS, half3 normalWS, half3 viewDirectionWS,
     half fogFactor, half3 bakedGI, half3 diffuse, half4 specularGloss, half shininess, half3 emission, half alpha)
 {
-    half3 lightDirection;
-
     Light mainLight = GetMainLight(positionWS);
     half3 indirectDiffuse = SubtractDirectMainLightFromLightmap(mainLight, normalWS, bakedGI);
 

ScriptableRenderPipeline/LightweightPipeline/LWRP/ShaderLibrary/LightweightPassMeta.hlsl

         // d3d9 shader compiler doesn't like NaNs and infinity.
         unity_OneOverOutputBoost = saturate(unity_OneOverOutputBoost);
 
-        // Disable pow(f, e) warning will not work with negative values
-#pragma warning (disable : 3571)
-#pragma warning (enable : 3571)
     }
     if (unity_MetaFragmentControl.y)
     {