aco: move s_andn2_b64 instructions out of the p_discard_if

aco: move s_andn2_b64 instructions out of the p_discard_if

And use a new p_discard_early_exit instruction. This fixes some cases
where a definition having the same register as an operand causes issues.

v2: rename instruction to p_exit_early_if
v2: modify the existing instruction instead of creating a new one
v3: merge the "i == num - 1" IFs

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>