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

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

compiler/testData/diagnostics/tests/smartCasts/varnotnull/whileTrueWithBracketSet.kt

@@ -0,0 +1,13 @@
+fun x(): Boolean { return true }
+
+public fun foo(pp: String?): Int {
+    var p = pp
+    while(true) {
+        p!!.length()
+        if (x()) break
+        (((p))) = null
+    }
+    // Smart cast is NOT possible here
+    // (we could provide it but p = null makes it much harder)
+    return p<!UNSAFE_CALL!>.<!>length()
+}