Rename the VOID type to INVALID and clean up obsolete comments.

The VOID type isn't used for anything resembling what "void" means in C,
so rename it to INVALID to avoid confusion.

lib/codegen/src/verifier/mod.rs

@@ -1106,8 +1106,8 @@ impl<'a> Verifier<'a> {
             ctrl_type
         } else {
             // Non-polymorphic instructions don't check the controlling type variable, so `Option`
-            // is unnecessary and we can just make it `VOID`.
-            types::VOID
+            // is unnecessary and we can just make it `INVALID`.
+            types::INVALID
         };
 
         // Typechecking instructions is never fatal