Rollforward of rollback:

Reinstate the use of integral-exponent power function MathUtil::IPow, but make sure to use a floating point base, so as to compute the result using floating point arithmetic. This behaviour is equivalent to, but faster than, std::pow.

Note that care must be taken to convert the base to double, which we effect by providing an explicit template type argument for MathUtil::IPow.

PiperOrigin-RevId: 211290304