backend: add tests for try-catch-finally and throwing exceptions

also add some tests for bugs with unreachable code handling and variables

backend.native/tests/codegen/dataflow/scope1.kt

@@ -0,0 +1,9 @@
+var b = true
+
+fun main(args: Array<String>) {
+    var x = 1
+    if (b) {
+        var x = 2
+    }
+    println(x)
+}