KT-14258 Optimize accesses to properties defined into companion

- Use direct access to property defined into companion object when
it is possible rather than always use an accessor to access the
property.
- Use direct access will speedup runtime performance.
- Avoid to generate useless accessors for companion properties.

Fix of https://youtrack.jetbrains.com/issue/KT-14258

compiler/testData/writeFlags/function/classObjectPrivate/privateVar.kt

@@ -4,6 +4,8 @@ class Foo {
 
   companion object {
     private var test = "String"
+      // Custom setter is needed, otherwise no need to generate setTest
+      set(v) { field = v }
   }
 }