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/nestedLoops.kt

@@ -0,0 +1,19 @@
+fun x(): Boolean { return true }
+
+public fun foo(qq: String?): Int {
+    var q = qq
+    while(true) {
+        q!!.length()
+        var r = q
+        do {
+            var p = r
+            do {
+                // p = r, r = q and q is not null
+                <!DEBUG_INFO_SMARTCAST!>p<!>.length()
+            } while (!x())
+        } while (<!SENSELESS_COMPARISON!>r == null<!>) // r = q and q is not null
+        if (!x()) break
+    }
+    // Smart cast is possible
+    return <!DEBUG_INFO_SMARTCAST!>q<!>.length()
+}