nir/algebraic: Optimize some u2f of bfi

v2: Fix a copy-and-paste bug s/('find_lsb', a)/a/ in the patterns. See
piglit!819.

DG2, Tiger Lake, Ice Lake, Skylake, and Broadwell had similar results (Ice Lake shown)
total instructions in shared programs: 20570063 -> 20570033 (<.01%)
instructions in affected programs: 452 -> 422 (-6.64%)
helped: 30 / HURT: 0

total cycles in shared programs: 902118723 -> 902118781 (<.01%)
cycles in affected programs: 1762 -> 1820 (3.29%)
helped: 0 / HURT: 29

DG2, Tiger Lake, Ice Lake, and Skylake had similar results (Ice Lake shown)
Totals:
Instrs: 152819969 -> 152819500 (-0.00%)
Cycles: 15014628652 -> 15014627187 (-0.00%); split: -0.00%, +0.00%

Totals from 469 (0.07% of 662497) affected shaders:
Instrs: 7644 -> 7175 (-6.14%)
Cycles: 31787 -> 30322 (-4.61%); split: -4.90%, +0.29%

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19968>

src/compiler/nir/nir_opt_algebraic.py

@@ -1385,6 +1385,15 @@ optimizations.extend([
    (('uror@16', a, b), ('ior', ('ushr', a, b), ('ishl', a, ('isub', 16, b))), 'options->lower_rotate'),
    (('uror@32', a, b), ('ior', ('ushr', a, b), ('ishl', a, ('isub', 32, b))), 'options->lower_rotate'),
    (('uror@64', a, b), ('ior', ('ushr', a, b), ('ishl', a, ('isub', 64, b))), 'options->lower_rotate'),
+
+   # Because 'a' is a positive power of two, the result of the bfi is either 0
+   # or 'a' depending on whether or not 'b' is odd.  Use 'b&1' for the zero
+   # value to help platforms that can't have two constants in a bcsel.
+   (('u2f32', ('bfi', '#a(is_pos_power_of_two)', b, 0)),
+    ('bcsel', ('ieq', ('iand', b, 1), 0), ('iand', b, 1), ('u2f', a))),
+   (('u2f', ('bfi', '#a(is_pos_power_of_two)', b, 0)),
+    ('bcsel', ('ieq', ('iand', b, 1), 0), 0, ('u2f', a))),
+
    # Exponential/logarithmic identities
    (('~fexp2', ('flog2', a)), a), # 2^lg2(a) = a
    (('~flog2', ('fexp2', a)), a), # lg2(2^a) = a