tree-optimization/118558 - fix alignment compute with VMAT_CONTIGUOUS_REVERSE

There are calls to dr_misalignment left that do not correct for the
offset (which is vector type dependent) when the stride is negative.
Notably vect_known_alignment_in_bytes doesn't allow to pass through
such offset which the following adds (computing the offset in
vect_known_alignment_in_bytes would be possible as well, but the
offset can be shared as seen).  Eventually this function could go away.

This leads to peeling for gaps not considerd, nor shortening of the
access applied which is what fixes the testcase on x86_64.

	PR tree-optimization/118558
	* tree-vectorizer.h (vect_known_alignment_in_bytes): Pass
	through offset to dr_misalignment.
	* tree-vect-stmts.cc (get_group_load_store_type): Compute
	offset applied for negative stride and use it when querying
	alignment of accesses.
	(vectorizable_load): Likewise.

	* gcc.dg/vect/pr118558.c: New testcase.