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/inference.kt

@@ -0,0 +1,21 @@
+// See KT-969
+fun f() {
+  var s: String?
+  s = "a"
+  var s1 = "" // String – ?
+  if (<!SENSELESS_COMPARISON!>s != null<!>) {    // Redundant
+    s1.length()
+    // We can do smartcast here and below
+    s1 = <!DEBUG_INFO_SMARTCAST!>s<!>.toString() // return String?
+    s1.length()
+    s1 = <!DEBUG_INFO_SMARTCAST!>s<!>
+    s1.length()
+    // It's just an assignment without smartcast
+    val s2 = s
+    // But smartcast can be done here
+    <!DEBUG_INFO_SMARTCAST!>s2<!>.length()
+    // And also here
+    val s3 = <!DEBUG_INFO_SMARTCAST!>s<!>.toString()
+    s3.length()
+  }
+}