nir/algebraic: Add some optimizations for D3D-style Booleans

nir/algebraic: Add some optimizations for D3D-style Booleans

D3D Booleans use a 32-bit 0/-1 representation.  Because this previously
matched NIR exactly, we didn't have to really optimize for it.  Now that
we have 1-bit Booleans, we need some specific optimizations to chew
through the D3D12-style Booleans.

Shader-db results on Kaby Lake:

    total instructions in shared programs: 15136811 -> 14967944 (-1.12%)
    instructions in affected programs: 2457021 -> 2288154 (-6.87%)
    helped: 8318
    HURT: 10

    total cycles in shared programs: 373544524 -> 359701825 (-3.71%)
    cycles in affected programs: 151029683 -> 137186984 (-9.17%)
    helped: 7749
    HURT: 682

    total loops in shared programs: 4431 -> 4399 (-0.72%)
    loops in affected programs: 32 -> 0
    helped: 21
    HURT: 0

    total spills in shared programs: 10290 -> 10051 (-2.32%)
    spills in affected programs: 2532 -> 2293 (-9.44%)
    helped: 18
    HURT: 18

    total fills in shared programs: 22203 -> 21732 (-2.12%)
    fills in affected programs: 3319 -> 2848 (-14.19%)
    helped: 18
    HURT: 18

Note that a large chunk of the improvement fixing regressions caused by
switching to 1-bit Booleans.  Previously, our ability to optimize D3D
booleans was improved by using the D3D representation directly in NIR.
Now that NIR does 1-bit bools, we need a few more optimizations.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>