Henrik Gramner authored Jan 29, 2021 and Henrik Gramner committed Feb 15, 2021

Not having a quantizer matrix is the most common case, so it's
worth having a separate code path for it that eliminates some
calculations and table lookups.

Without a qm, not only can we skip calculating dq * qm, but only
Exp-Golomb-coded coefficients will have the potential to overflow,
so we can also skip clipping for the vast majority of coefficients.