nir: Simplify a bcsel to logical-or

nir: Simplify a bcsel to logical-or

Oddly, this did not affect the shader where I first noticed the pattern.
That particular shader doesn't get its if-statement converted to a bcsel
because there are two assignments in the else-statement.  This led to me
submitting https://bugs.freedesktop.org/show_bug.cgi?id=94747.

shader-db results:

Sandy Bridge
total instructions in shared programs: 8467384 -> 8467069 (-0.00%)
instructions in affected programs: 36594 -> 36279 (-0.86%)
helped: 46
HURT: 0

total cycles in shared programs: 117573448 -> 117568518 (-0.00%)
cycles in affected programs: 339114 -> 334184 (-1.45%)
helped: 46
HURT: 0

Ivy Bridge / Haswell / Broadwell / Skylake:
total instructions in shared programs: 7774258 -> 7773999 (-0.00%)
instructions in affected programs: 30874 -> 30615 (-0.84%)
helped: 46
HURT: 0

total cycles in shared programs: 65739190 -> 65734530 (-0.01%)
cycles in affected programs: 180380 -> 175720 (-2.58%)
helped: 45
HURT: 1

No change on G45 or Ironlake.

I also tried these expressions, but none of them affected any shaders in
shader-db:

   (('bcsel', a, 'a@bool', 'b@bool'), ('ior', a, b)),
   (('bcsel', a, 'b@bool', False),    ('iand', a, b)),
   (('bcsel', a, 'b@bool', 'a@bool'), ('iand', a, b)),

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>