Add TranslationState::in_unreachable_code().

Move an unreachable code test and sanity check into this method.

lib/wasm/src/code_translator.rs

@@ -118,11 +118,7 @@ pub fn translate_function_body(
             let parser_state = parser.read();
             match *parser_state {
                 ParserState::CodeOperator(ref op) => {
-                    debug_assert!(
+                    if state.in_unreachable_code() {
-                        state.phantom_unreachable_stack_depth == 0 ||
-                            state.real_unreachable_stack_depth > 0
-                    );
-                    if state.real_unreachable_stack_depth > 0 {
                         translate_unreachable_operator(op, &mut builder, &mut state)
                     } else {
                         translate_operator(

lib/wasm/src/state.rs

@@ -200,4 +200,14 @@ impl TranslationState {
             reachable: false,
         });
     }
+
+    /// Test if the translation state is currently in unreachable code.
+    pub fn in_unreachable_code(&self) -> bool {
+        if self.real_unreachable_stack_depth > 0 {
+            true
+        } else {
+            debug_assert_eq!(self.phantom_unreachable_stack_depth, 0, "in reachable code");
+            false
+        }
+    }
 }