vect: Reject non-byte offsets for gather/scatters [PR107346]

The ada failure reported in the PR was being caused by vect_check_gather_scatter
failing to deal with bit offsets that weren't multiples of BITS_PER_UNIT. This
patch makes vect_check_gather_scatter reject memory accesses with such offsets.

gcc/ChangeLog:

	PR tree-optimization/107346
	* tree-vect-data-refs.cc (vect_check_gather_scatter): Reject offsets
	that aren't multiples of BITS_PER_UNIT.