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/ifVarNotNullAnd.kt

@@ -0,0 +1,13 @@
+// See KT-5737
+fun get(): String? {
+    return "abc"
+}
+
+fun foo(): Int {
+    var ss:String? = get()
+
+    return if (ss != null && <!DEBUG_INFO_SMARTCAST!>ss<!>.length() > 0)
+        1
+    else
+        0
+}