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/variables/initialization.kt

@@ -0,0 +1,8 @@
+fun foo() {
+    var v: Any = "xyz"
+    // It is possible in principle to provide smart cast here
+    // but now we decide that v is Any
+    v.<!UNRESOLVED_REFERENCE!>length<!>()
+    v = 42
+    v.<!UNRESOLVED_REFERENCE!>length<!>()
+}