backend: add tests:

* test for uninitialized vals
* test initializers3: global `var` of reference type
* expression_as_statement (disabled): using type operator as statement

Also enable test break1.

backend.native/tests/codegen/dataflow/uninitialized_val.kt

@@ -0,0 +1,17 @@
+fun foo(b: Boolean): Int {
+    val x: Int
+    if (b) {
+        x = 1
+    } else {
+        x = 2
+    }
+
+    return x
+}
+
+fun main(args: Array<String>) {
+    val uninitializedUnused: Int
+
+    println(foo(true))
+    println(foo(false))
+}