nir/algebraic: Add some max/min optimizations with 3 variables

Specifically, ARB assembly shaders with code like

    SLT    r0, r0, c[0].xxxx;
    ...
    KIL    r0.xyzx;

can result in this pattern.  The other cases (e.g., 'KIL r0.xxxx' and
'KIL r0.xyxx') are handled by existing patterns.

Reviewed-by: Matt Turner <mattst88@gmail.com>

All Intel platforms had similar results. (Tiger Lake shown)
total instructions in shared programs: 21050098 -> 21050065 (<.01%)
instructions in affected programs: 2062 -> 2029 (-1.60%)
helped: 31
HURT: 1
helped stats (abs) min: 1 max: 3 x̄: 1.10 x̃: 1
helped stats (rel) min: 1.14% max: 4.35% x̄: 1.89% x̃: 1.69%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.65% max: 0.65% x̄: 0.65% x̃: 0.65%
95% mean confidence interval for instructions value: -1.23 -0.84
95% mean confidence interval for instructions %-change: -2.12% -1.50%
Instructions are helped.

total cycles in shared programs: 855105466 -> 855105055 (<.01%)
cycles in affected programs: 50136 -> 49725 (-0.82%)
helped: 33
HURT: 0
helped stats (abs) min: 3 max: 22 x̄: 12.45 x̃: 12
helped stats (rel) min: 0.13% max: 1.57% x̄: 0.86% x̃: 0.92%
95% mean confidence interval for cycles value: -13.78 -11.13
95% mean confidence interval for cycles %-change: -0.97% -0.76%
Cycles are helped.

No fossil-db changes on any Intel platform.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9122>

src/compiler/nir/nir_opt_algebraic.py

@@ -554,6 +554,12 @@ optimizations.extend([
    (('fmin', ('fmin', a, b), b), ('fmin', a, b)),
    (('umin', ('umin', a, b), b), ('umin', a, b)),
    (('imin', ('imin', a, b), b), ('imin', a, b)),
+   (('fmax', ('fmax', ('fmax', a, b), c), a), ('fmax', ('fmax', a, b), c)),
+   (('umax', ('umax', ('umax', a, b), c), a), ('umax', ('umax', a, b), c)),
+   (('imax', ('imax', ('imax', a, b), c), a), ('imax', ('imax', a, b), c)),
+   (('fmin', ('fmin', ('fmin', a, b), c), a), ('fmin', ('fmin', a, b), c)),
+   (('umin', ('umin', ('umin', a, b), c), a), ('umin', ('umin', a, b), c)),
+   (('imin', ('imin', ('imin', a, b), c), a), ('imin', ('imin', a, b), c)),
 ])
 
 # Integer sizes