aco: optimize add/sub(a, cndmask(b, 0, 1, cond)) -> addc/subbrev_co(0, a, b)

aco: optimize add/sub(a, cndmask(b, 0, 1, cond)) -> addc/subbrev_co(0, a, b)

v2: outline into a separate function and also optimize additions (by Daniel Schürmann)

Totals from affected shaders: (VEGA)
SGPRS: 938888 -> 941496 (0.28 %)
VGPRS: 832068 -> 831532 (-0.06 %)
Spilled SGPRs: 618 -> 618 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 3696 -> 3696 (0.00 %) dwords per thread
Code Size: 72893900 -> 72558928 (-0.46 %) bytes
LDS: 18201 -> 18201 (0.00 %) blocks
Max Waves: 64256 -> 64268 (0.02 %)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Co-authored-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4419>