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/try/finally6.kt

@@ -0,0 +1,16 @@
+fun main(args : Array<String>) {
+    println(foo())
+}
+
+fun foo(): Int {
+    try {
+        println("Done")
+        return 0
+    } finally {
+        println("Finally")
+        return 1
+    }
+
+    println("After")
+    return 2
+}