I think WUFFS "iterate loops" might help. WUFFS requires that processing a chunk of N items has code to process one at a time, which means it'll work for any N. However you can optionally provide specialisations for doing K at a time and the compiler is responsible for carving the input up as appropriate so e.g. N = K + K + 1 + 1 + 1 your K-at-a-time code runs twice, the extras are handled 1-at-a-time.

So this divides up the problem, the compiler can vectorize your 8-at-a-time code without needing to handle edge cases where N isn't a multiple of 8, and if a later pass notices we actually never end up using those edge cases they're dead code, if it doesn't they're just a rarely-taken branch once.