Move bounds check out of loop in fwd_txfm2d_daala (#1851)
Move bounds check out of loop in fwd_txfm2d_daala (#1851)

Provides a very minor performance improvement.
In order to auto-vectorize, more bounds checks
will need to be elided, which are more difficult to remove
without unsafe code.
xiph/rav1e,xiph/rav1e