Backport "aco: fix unconditional demote_to_helper"

This patch fixes an out-of-bounds access on p_exit_early and binds the exec register to the correct operand.

Fixes: 2ea9e59e ('aco: move s_andn2_b64 instructions out of the p_discard_if') Reviewed-by: Rhys Perry pendingchaos02@gmail.com Tested-by: Marge Bot !3347 (merged) Part-of: !3347 (merged)