x86: Fix AVX2 inverse identity transform overflow/clipping
x86: Fix AVX2 inverse identity transform overflow/clipping

The coefficients after the first (8-bit) 1D identity transform may
require more than 16 bits precision before downshifting in some cases,
and they may also need to be clipped to int16_t after downshifting.

xiph/rav1e,xiph/rav1e