glsl: Optimize pow(2, x) --> exp2(x).

On Haswell, POW takes 24 cycles, while EXP2 only takes 14.  Plus, using
POW requires putting 2.0 in a register, while EXP2 doesn't.

I believe that EXP2 will be faster than POW on basically all GPUs, so
it makes sense to optimize it.

Looking at the savage2 subset of shader-db:
total instructions in shared programs: 113225 -> 113179 (-0.04%)
instructions in affected programs:     2139 -> 2093 (-2.15%)
instances of 'math pow':               795 -> 749 (-6.14%)
instances of 'math exp':               389 -> 435 (11.8%)

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

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