quartus 7.1 SP1 produces WRONG rtl !

An always construct contains a sequential block of code. That is, the statements execute in a particular sequence or order. A blocking assignment updates the value of the target BEFORE any of the following statements execute. So if you reverse the order of the assignments, you don't update cs to 1'bz prior to using it in your expression. Even so, it's not a safe transformation. If the always construct re-executed for any reason, you'd get the result you don't want.