aco: consider pseudo-instructions reading exec in needs_exec_mask()

No matter the format, this should return true if the instruction has an exec operand.

Otherwise, eliminate_useless_exec_writes_in_block() could remove an exec write in a block if it's successor begins with:

 s2: %3737:s[8-9] = p_parallelcopy %0:exec
 s2: %0:exec,  s1: %3738:scc = s_wqm_b64 %3737:s[8-9]