glsl: Optimize 1/exp(x) into exp(-x).

glsl: Optimize 1/exp(x) into exp(-x).

Lots of shaders divide by exp2(...) which we turn into a multiplication
by the reciprocal. We can avoid the reciprocal by simply negating exp2's
argument.

total instructions in shared programs: 5947154 -> 5946695 (-0.01%)
instructions in affected programs:     118661 -> 118202 (-0.39%)
helped:                                380

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>