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/whileTrue.kt

@@ -0,0 +1,13 @@
+fun x(): Boolean { return true }
+
+public fun foo(pp: String?): Int {
+    var p = pp
+    while(true) {
+        p!!.length()
+        if (x()) break
+        p = null
+    }
+    // Smart cast is NOT possible here
+    // (we could provide it but p = null makes it much harder)
+    return p<!UNSAFE_CALL!>.<!>length()
+}