Elide bounds checks in cdef_dist_wxh_8x8 inner loop (#1984)
Elide bounds checks in cdef_dist_wxh_8x8 inner loop (#1984)

Although rustc still fails to autovectorize this function,
eliding the extra bounds check branches
gains 2.5% encode speed at speed 6
due to how hot this function is.
xiph/rav1e,xiph/rav1e