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/try1.kt

@@ -0,0 +1,9 @@
+fun main(args : Array<String>) {
+    val x = try {
+        5
+    } catch (e: Throwable) {
+        6
+    }
+
+    println(x)
+}