nir: Do basic constant reassociation.

Many shaders contain expression trees of the form:

    const_1 * (value * const_2)

Reorganizing these to

    (const_1 * const_2) * value

will allow constant folding to combine the constants.  Sometimes, these
constants are 2 and 0.5, so we can remove a multiply altogether.  Other
times, it can create more immediate constants, which can actually hurt.

Finding a good balance here is tricky.  While much more could be done,
this simple patch seems to have a lot of positive benefit while having
a low downside.

shader-db results on Broadwell:

total instructions in shared programs: 8963768 -> 8961369 (-0.03%)
instructions in affected programs: 438318 -> 435919 (-0.55%)
helped: 1502
HURT: 245

total cycles in shared programs: 71527354 -> 71421516 (-0.15%)
cycles in affected programs: 11541788 -> 11435950 (-0.92%)
helped: 3445
HURT: 1224

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Eduardo Lima Mitev <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

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