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/varnotnull/varChangedInLoop.kt

@@ -0,0 +1,9 @@
+public fun foo() {
+    var i: Int? = 1
+    if (i != null) {
+        while (i != 10) {
+            <!UNUSED_CHANGED_VALUE!>i<!UNSAFE_CALL!>++<!><!>      // Here smart cast should not be performed due to a successor
+            i = null
+        }
+    }
+}