FE: Optimize ConeFlexibleType's hashCode/equals for trivial types

Author: dzharkov

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeTypes.kt

@@ -143,14 +143,22 @@ open class ConeFlexibleType(
         if (other !is ConeFlexibleType) return false
 
         if (lowerBound != other.lowerBound) return false
+
+        if (isTrivial && other.isTrivial) return true
+
         if (upperBound != other.upperBound) return false
 
         return true
     }
 
     final override fun hashCode(): Int {
         var result = lowerBound.hashCode()
-        result = 31 * result + upperBound.hashCode()
+        // We don't use `upperBound.hashCode()` because it might lead to performance loss for trivial types.
+        // While doing something like `31 * lowerBoundResult + Boolean.hashCode(true/* markedNullable */)`
+        // to replicate `upperBound.hashCode()` behavior seems too fragile.
+        // But we want the result was different from just lowerBound's one,
+        // so we add a beautiful though random prime number.
+        result = 31 * result + 2999
         return result
     }
 }