Fix warnings (causing CI failures) with new Rust beta.

- Panic messages must now be string literals (we used `format!()` in
  many places; `panic!()` can take format strings directly).
- Some dead enum options with EVEX encoding stuff in old x86 backend.
  This will go away soon and/or be moved to the new backend anyway, so
  let's silence the warning for now.
- A few other misc warnings.

cranelift/wasm/tests/wasm_testsuite.rs

@@ -90,7 +90,7 @@ fn handle_module(data: Vec<u8>, flags: &Flags, return_mode: ReturnMode) {
 
     for func in dummy_environ.info.function_bodies.values() {
         verifier::verify_function(func, &*isa)
-            .map_err(|errors| panic!(pretty_verifier_error(func, Some(&*isa), None, errors)))
+            .map_err(|errors| panic!("{}", pretty_verifier_error(func, Some(&*isa), None, errors)))
             .unwrap();
     }
 }