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

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

compiler/testData/diagnostics/tests/Casts.kt

@@ -8,7 +8,7 @@ fun test() : Unit {
   y <!USELESS_CAST!>as Int<!> : Int
   x <!USELESS_CAST!>as Int?<!> : Int?
   y <!USELESS_CAST_STATIC_ASSERT_IS_FINE!>as Int?<!> : Int?
-  x as? Int : Int?
+  x <!USELESS_CAST_STATIC_ASSERT_IS_FINE!>as? Int<!> : Int?
   y <!USELESS_CAST!>as? Int<!> : Int?
   x <!USELESS_CAST!>as? Int?<!> : Int?
   y <!USELESS_CAST_STATIC_ASSERT_IS_FINE!>as? Int?<!> : Int?

compiler/testData/diagnostics/tests/UnusedVariables.kt

@@ -68,7 +68,7 @@ class MyTest() {
         }
         else {
             a = "ss"
-            doSmth(a as String)
+            doSmth(<!DEBUG_INFO_SMARTCAST!>a<!>)
         }
         doSmth(a)
 

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()
+}

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

@@ -0,0 +1,4 @@
+package
+
+public fun foo(/*0*/ pp: kotlin.String?): kotlin.Int
+internal fun x(): kotlin.Boolean

compiler/testData/diagnostics/tests/smartCasts/varnotnull/whileTrueWithBrackets.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()
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/whileTrueWithBrackets.txt

@@ -0,0 +1,4 @@
+package
+
+public fun foo(/*0*/ pp: kotlin.String?): kotlin.Int
+internal fun x(): kotlin.Boolean