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/varCapturedInSafeClosure.kt

@@ -0,0 +1,15 @@
+public fun foo() {
+    var s: String? = ""
+    fun closure(): Int {
+        if (s == null) {
+            return -1
+        } else {
+            return 0
+        }
+    }
+    if (s != null) {
+        System.out.println(closure())
+        // Smart cast is possible but closure makes it harder to understand
+        System.out.println(s<!UNSAFE_CALL!>.<!>length())   
+    }
+}