glsl: Rebalance expression trees that are reduction operations.

glsl: Rebalance expression trees that are reduction operations.

The intention of this pass was to give us better instruction scheduling
opportunities, but it unexpectedly reduced some instruction counts as
well:

total instructions in shared programs: 1666639 -> 1666073 (-0.03%)
instructions in affected programs:     54612 -> 54046 (-1.04%)
(and trades 4 SIMD16 programs in SS3)