Do not double check for already decoded expn_id to avoid races

Author: zetanumbers

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1560,7 +1560,6 @@ impl<'a> CrateMetadataRef<'a> {
     }
 
     fn expn_hash_to_expn_id(self, sess: &Session, index_guess: u32, hash: ExpnHash) -> ExpnId {
-        debug_assert_eq!(ExpnId::from_hash(hash), None);
         let index_guess = ExpnIndex::from_u32(index_guess);
         let old_hash = self.root.expn_hashes.get(self, index_guess).map(|lazy| lazy.decode(self));