[DAG] Don't fold (zext (bitop (load x), cst)) -> (bitop (zextload x), (zext...

[DAG] Don't fold (zext (bitop (load x), cst)) -> (bitop (zextload x), (zext cst)) if the zext is free

Prevents an infinite loop if we've been trying to narrow the bitop to a more preferable type