Avoid explicitly using `roughness` in the IBL code

Doesn't seem to help as the compiler still caches roughness, but it's a start.

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/ImageBasedLighting.hlsl

 }
 
 // Ref: "Moving Frostbite to PBR", p. 69.
-real3 GetSpecularDominantDir(real3 N, real3 R, real roughness, real NdotV)
+real3 GetSpecularDominantDir(real3 N, real3 R, real perceptualRoughness, real NdotV)
-    real a = 1.0 - roughness;
+    real p = perceptualRoughness;
+    real a = 1.0 - p * p;
-    real lerpFactor = (s + roughness) * a;
+    real lerpFactor = (s + p * p) * a;
-    real lerpFactor = (s + roughness) * saturate(a * a + lerp(0.0, a, NdotV * NdotV));
+    real lerpFactor = (s + p * p) * saturate(a * a + lerp(0.0, a, NdotV * NdotV));
 #endif
 
     // The result is not normalized as we fetch in a cubemap
 // To simulate the streching of highlight at grazing angle for IBL we shrink the roughness
 // which allow to fake an anisotropic specular lobe.
 // Ref: http://www.frostbite.com/2015/08/stochastic-screen-space-reflections/ - slide 84
-real AnisotropicStrechAtGrazingAngle(real roughness, real perceptualRoughness, real NdotV)
+real AnisotropicStrechAtGrazingAngle(real perceptualRoughness, real NdotV)
-    return roughness * lerp(saturate(NdotV * 2.0), 1.0, perceptualRoughness);
+    real p = perceptualRoughness;
+    return p * lerp(saturate(NdotV * 2.0) * p, p, p);
 }
 
 // ----------------------------------------------------------------------------

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/Lit.hlsl

     // This is a ad-hoc tweak to better match reference of anisotropic GGX.
     // TODO: We need a better hack.
     preLightData.iblPerceptualRoughness *= saturate(1.2 - abs(bsdfData.anisotropy));
-    float iblRoughness = PerceptualRoughnessToRoughness(preLightData.iblPerceptualRoughness);
-    preLightData.iblR = GetSpecularDominantDir(N, iblR, iblRoughness, NdotV);
+    preLightData.iblR = GetSpecularDominantDir(N, iblR, preLightData.iblPerceptualRoughness, NdotV);
 
 #ifdef LIT_USE_GGX_ENERGY_COMPENSATION
     // Ref: Practical multiple scattering compensation for microfacet models.