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/forEach.kt

@@ -0,0 +1,16 @@
+data class SomeObject(val n: SomeObject?) {
+    fun doSomething() {}
+    fun next(): SomeObject? = n    
+}
+
+
+fun list(start: SomeObject): SomeObject {
+    var e: SomeObject? = start
+    for (i in 0..42) {
+        // Unsafe calls because of nullable e at the beginning
+        e<!UNSAFE_CALL!>.<!>doSomething()
+        e = e<!UNSAFE_CALL!>.<!>next()
+    }
+    // Smart cast is not possible here due to next()
+    return <!TYPE_MISMATCH!>e<!>
+}