Add early-stage optimization crate (#556)

* Add simple constant folding and folding tests

lib/codegen/src/lib.rs

@@ -104,12 +104,12 @@ mod nan_canonicalization;
 mod partition_slice;
 mod postopt;
 mod predicates;
-mod preopt;
 mod ref_slice;
 mod regalloc;
 mod result;
 mod scoped_hash_map;
 mod simple_gvn;
+mod simple_preopt;
 mod stack_layout;
 mod topo_order;
 mod unreachable_code;