Review fixes for mutable variable smart casting (mostly refactoring).

DataFlowValue.isPredictable() introduced instead of isStableIdentifier().
Parenthesized assignments are treated correctly.

idea/idea-completion/src/org/jetbrains/kotlin/idea/completion/DataFlowInfoUtilForCompletion.kt

@@ -24,7 +24,7 @@ import org.jetbrains.kotlin.descriptors.PackageViewDescriptor
 
 fun renderDataFlowValue(value: DataFlowValue): String? {
     // If it is not a stable identifier, there's no point in rendering it
-    if (!value.isStableIdentifier() && !value.isLocalVariable()) return null
+    if (!value.isPredictable()) return null
 
     fun renderId(id: Any?): String? {
         return when (id) {