aco/optimizer: optimize s_and(exec, s_and(x, y)) more aggressively

aco/optimizer: optimize s_and(exec, s_and(x, y)) more aggressively

It is enough if either of the two operands is respecting the
current exec mask.

Totals from 1680 (1.25% of 134913) affected shaders: (GFX10.3)
CodeSize: 3929436 -> 3922372 (-0.18%); split: -0.18%, +0.00%
Instrs: 730305 -> 728536 (-0.24%); split: -0.24%, +0.00%
Latency: 6839314 -> 6835154 (-0.06%); split: -0.07%, +0.01%
InvThroughput: 1371351 -> 1371267 (-0.01%); split: -0.01%, +0.00%
SClause: 32819 -> 32802 (-0.05%); split: -0.09%, +0.04%
Copies: 33264 -> 33271 (+0.02%); split: -0.01%, +0.03%

Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18253>