ac/nir: micro-optimize boolean expression

ac/nir: micro-optimize boolean expression

Ignoring SCC spilling, the old version is probably faster because this
mixes uniform and divergent booleans.

fossil-db (navi21):
Totals from 61167 (45.10% of 135636) affected shaders:
Instrs: 29961899 -> 29932551 (-0.10%)
CodeSize: 157407028 -> 157289636 (-0.07%)
Latency: 139671953 -> 139625186 (-0.03%); split: -0.03%, +0.00%
InvThroughput: 21221097 -> 21220756 (-0.00%)
SClause: 750438 -> 750439 (+0.00%)
Copies: 2672846 -> 2582332 (-3.39%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19340>