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/controlflow/unreachable1.kt

@@ -0,0 +1,8 @@
+fun main(args: Array<String>) {
+    println(foo())
+}
+
+fun foo(): Int {
+    return 1
+    println("After return")
+}