Fixed bug in <when> codegen + test

backend.native/tests/codegen/branching/when_with_try1.kt

@@ -0,0 +1,22 @@
+package codegen.branching.when_with_try1
+
+import kotlin.test.*
+
+@Test fun runTest() {
+    println(foo(Any()))
+    println(foo("zzz"))
+    println(foo("42"))
+}
+
+fun foo(value: Any): Int? {
+    if (value is CharSequence) {
+        try {
+            return value.toString().toInt()
+        } catch (e: NumberFormatException) {
+            return null
+        }
+    }
+    else {
+        return null
+    }
+}