Somehow, folding calls to log2() with a constant was missing.
I'm not really happy with adding more uses of ConstantFoldFP/ConstantFoldBinaryFP; they're flawed because they produce results that depend on the host's libm implementation. But I guess there isn't any reason to support log and not log2.
I think a few of the functions we use ConstantFoldFP with actually already have implementations in APFloat: floor, ceil, round, fabs, fmod.
I'm not sure "C99" is really relevant; I don't think we support any hosts which don't have C99 libm, anyway.