Track wasm reachability explicitly.

Maintain an explicit "reachable" flag when decoding wasm. Push placeholder
frames on the control-flow stack instead of just maintaining a count of
the stack depth in unreachable code, so that we can whether If blocks
have Elses, and whether block exits are branched to, in all contexts.

Fixes #217.

lib/wasm/src/func_translator.rs

@@ -206,9 +206,11 @@ fn parse_function_body<FE: FuncEnvironment + ?Sized>(
     //
     // If the exit block is unreachable, it may not have the correct arguments, so we would
     // generate a return instruction that doesn't match the signature.
-    debug_assert!(builder.is_pristine());
-    if !builder.is_unreachable() {
-        builder.ins().return_(&state.stack);
+    if state.reachable {
+        debug_assert!(builder.is_pristine());
+        if !builder.is_unreachable() {
+            builder.ins().return_(&state.stack);
+        }
     }
 
     // Discard any remaining values on the stack. Either we just returned them,