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/finally8.kt

@@ -0,0 +1,18 @@
+fun main(args : Array<String>) {
+    println(foo())
+}
+
+fun foo(): Int {
+    try {
+        try {
+            return 42
+        } finally {
+            println("Finally 1")
+        }
+    } finally {
+        println("Finally 2")
+    }
+
+    println("After")
+    return 2
+}