Further optimize DiffuseSphereLightIrradiance()

Assets/ScriptableRenderPipeline/ShaderLibrary/AreaLighting.hlsl

 {
     float irradiance;
 
-    float sinSqOmega = saturate(1 - cosOmega * cosOmega);
+    float sinSqOmega = saturate(1 - cosOmega * cosOmega);
     float cosSqSigma = saturate(1 - sinSqSigma);
     float sinSqGamma = saturate(cosSqSigma / sinSqOmega);
     float cosSqGamma = saturate(1 - sinSqGamma);
         }
     }
 #else // Ref: Moving Frostbite to Physically Based Rendering, page 47 (2015).
+    float cosSqOmega = cosOmega * cosOmega;
+
-    if (cosOmega * cosOmega > sinSqSigma)
+    if (cosSqOmega > sinSqSigma)
-        float a = 1 / sinSqSigma - 1;
-        float w = rsqrt(a);
-        float x = sqrt(a);
-        float y = -x * cosOmega * rsqrt(sinSqOmega);
-        float z = sqrt(sinSqOmega * (1 - y * y));
+        float cotanOmega = cosOmega * rsqrt(1 - cosSqOmega);
-        irradiance = INV_PI * ((cosOmega * acos(y) - x * z) * sinSqSigma + atan(z * w));
+        float x = rcp(sinSqSigma) - 1;
+        float y = -cotanOmega * sqrt(x);
+        float z = sqrt(1 - cosSqOmega * rcp(sinSqSigma));
+
+        irradiance = INV_PI * ((cosOmega * acos(y) - z * sqrt(x)) * sinSqSigma + atan(z * rsqrt(x)));
     }
 #endif
     return max(irradiance, 0);