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/nestedDoWhile.kt

@@ -0,0 +1,15 @@
+fun x(): Boolean { return true }
+
+public fun foo(pp: String?, rr: String?): Int {
+    var p = pp
+    var r = rr
+    do {
+        do {
+            p!!.length()
+        } while (r == null)  
+    } while (!x())
+    // Auto cast possible
+    <!DEBUG_INFO_SMARTCAST!>r<!>.length()
+    // Auto cast possible
+    return <!DEBUG_INFO_SMARTCAST!>p<!>.length()
+}