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.

j2k/testData/fileOrElement/boxedType/Boxing.kt

@@ -5,8 +5,8 @@ class Boxing {
         var i: Int? = 0
         val n = 0.0f
         i = 1
-        var j = i!!
-        val k = i!! + 2
+        var j = i
+        val k = i + 2
         i = null
         j = i!!
     }