Consider the following search expression and NIR sequence:

    ('iadd', ('imul', a, b), b)

    ssa_2 = imul ssa_0, ssa_1
    ssa_3 = iadd ssa_2, ssa_0

The current algorithm is greedy and, the moment the imul finds a match,
it commits those variable names and returns success.  In the above
example, it maps a -> ssa_0 and b -> ssa_1.  When we then try to match
the iadd, it sees that ssa_0 is not b and fails to match.  The iadd
match will attempt to flip itself and try again (which won't work) but
it cannot ask the imul to try a flipped match.

This commit instead counts the number of commutative ops in each
expression and assigns an index to each.  It then does a loop and loops
over the full combinatorial matrix of commutative operations.  In order
to keep things sane, we limit it to at most 4 commutative operations (16
combinations).  There is only one optimization in opt_algebraic that
goes over this limit and it's the bitfieldReverse detection for some UE4
demo.

Shader-db results on Kaby Lake:

    total instructions in shared programs: 15310125 -> 15302469 (-0.05%)
    instructions in affected programs: 1797123 -> 1789467 (-0.43%)
    helped: 6751
    HURT: 2264

    total cycles in shared programs: 357346617 -> 357202526 (-0.04%)
    cycles in affected programs: 15931005 -> 15786914 (-0.90%)
    helped: 6024
    HURT: 3436

    total loops in shared programs: 4360 -> 4360 (0.00%)
    loops in affected programs: 0 -> 0
    helped: 0
    HURT: 0

    total spills in shared programs: 23675 -> 23666 (-0.04%)
    spills in affected programs: 235 -> 226 (-3.83%)
    helped: 5
    HURT: 1

    total fills in shared programs: 32040 -> 32032 (-0.02%)
    fills in affected programs: 190 -> 182 (-4.21%)
    helped: 6
    HURT: 2

    LOST:   18
    GAINED: 5

Reviewed-by: Thomas Helland <thomashelland90@gmail.com>