convert isOverrideHierarchyElement() to extension function

idea/src/org/jetbrains/kotlin/idea/hierarchy/overrides/KotlinOverrideHierarchyBrowser.kt

@@ -43,7 +43,7 @@ class KotlinOverrideHierarchyBrowser(
             )
 
     override fun isApplicableElement(psiElement: PsiElement): Boolean =
-            isOverrideHierarchyElement(psiElement)
+            psiElement.isOverrideHierarchyElement()
 
     override fun createHierarchyTreeStructure(typeName: String, psiElement: PsiElement): HierarchyTreeStructure? =
             if (typeName == MethodHierarchyBrowserBase.METHOD_TYPE) KotlinOverrideTreeStructure(myProject, psiElement) else null

idea/src/org/jetbrains/kotlin/idea/hierarchy/overrides/KotlinOverrideHierarchyProvider.kt

@@ -44,8 +44,8 @@ public class KotlinOverrideHierarchyProvider: HierarchyProvider {
     }
 
     private fun getOverrideHierarchyElement(element: PsiElement?): PsiElement?
-            = element?.getParentOfTypesAndPredicate() { isOverrideHierarchyElement(it) }
+            = element?.getParentOfTypesAndPredicate() { it.isOverrideHierarchyElement() }
 }
 
-fun isOverrideHierarchyElement(element: PsiElement): Boolean
-        = element is PsiMethod || element is JetNamedFunction || element is JetProperty
+fun PsiElement.isOverrideHierarchyElement(): Boolean
+        = this is PsiMethod || this is JetNamedFunction || this is JetProperty