Smart casts for local variables not captured in a closure and not changed in a loop, see #KT-3175.

isLocalVariable added. Assignment / initialization analysis.
Control whether a variable is changed in a loop at the beginning and at the end of the loop.
Control whether a variable is captured in a closure. #KT-3175  Fixed. #KT-2266  Fixed.
Tests for variable null safety and for variables is/as operations.
Loop / closure / variable property tests are included.
Old tests changed in accordance with KT-3175. In particular, all three of testSmartcastImpossible were fixed.

compiler/testData/diagnostics/tests/smartCasts/variables/ifVarIsChanged.kt

@@ -0,0 +1,11 @@
+public fun bar(s: String) {
+    System.out.println("Length of $s is ${s.length()}")
+}
+
+public fun foo() {
+    var s: Any = "not null"
+    if (s is String) {
+        s = 42
+        bar(<!TYPE_MISMATCH!>s<!>)
+    }
+}