nir/algebraic: Replace a bcsel of a b2f sources with a b2f(!(a || b))

nir/algebraic: Replace a bcsel of a b2f sources with a b2f(!(a || b))

I have not investigated the result of doing this during code
generation.  That should be possible, but it would be a bit more
effort.

All Gen6+ platforms had nearly identical results. (Skylake shown)
total cycles in shared programs: 370961508 -> 370961367 (<.01%)
cycles in affected programs: 5174 -> 5033 (-2.73%)
helped: 2
HURT: 0

Iron Lake and GM45 had similar results. (Iron Lake shown)
total instructions in shared programs: 8206587 -> 8206589 (<.01%)
instructions in affected programs: 1325 -> 1327 (0.15%)
helped: 0
HURT: 2

total cycles in shared programs: 187657422 -> 187657428 (<.01%)
cycles in affected programs: 11566 -> 11572 (0.05%)
helped: 0
HURT: 2

This change has almost no effect right now.  However, removing this
patch (but leaving the patch "intel/fs: Generate if instructions with
inverted conditions") after adding a patch that removes !(a < b) -> (a
>= b) optimizations (like
https://patchwork.freedesktop.org/patch/264787/) has the following
results on Skylake:

Skylake
total instructions in shared programs: 15071804 -> 15071806 (<.01%)
instructions in affected programs: 640 -> 642 (0.31%)
helped: 0
HURT: 2

total cycles in shared programs: 369914348 -> 369916569 (<.01%)
cycles in affected programs: 27900 -> 30121 (7.96%)
helped: 4
HURT: 15
helped stats (abs) min: 2 max: 112 x̄: 30.00 x̃: 3
helped stats (rel) min: 0.28% max: 12.28% x̄: 3.34% x̃: 0.40%
HURT stats (abs)   min: 2 max: 758 x̄: 156.07 x̃: 81
HURT stats (rel)   min: 0.20% max: 74.30% x̄: 16.29% x̃: 16.91%
95% mean confidence interval for cycles value: 12.68 221.11
95% mean confidence interval for cycles %-change: 3.09% 21.23%
Cycles are HURT.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>