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/unnecessaryWithMap.kt

@@ -0,0 +1,19 @@
+fun create(): Map<String, String> = null!!
+
+fun <K, V> Map<K, V>.iterator(): Iterator<Map.Entry<K, V>> = null!!
+
+fun <K, V> Map.Entry<K, V>.component1() = getKey()
+
+fun <K, V> Map.Entry<K, V>.component2() = getValue()
+
+class MyClass {
+    private var m: Map<String, String>? = null
+    fun foo(): Int {
+        var res = 0
+        m = create()
+        // See KT-7428
+        for ((k, v) in <!SMARTCAST_IMPOSSIBLE!>m<!>)
+            res += (k.length() + v.length())
+        return res
+    }
+}