vect: Fix bogus alignment assumption in alias checks [PR94994]

vect: Fix bogus alignment assumption in alias checks [PR94994]

This PR is about a case in which the vectoriser was feeding
incorrect alignment information to tree-data-ref.c, leading
to incorrect runtime alias checks.  The alignment was taken
from the TREE_TYPE of the DR_REF, which in this case was a
COMPONENT_REF with a normally-aligned type.  However, the
underlying MEM_REF was only byte-aligned.

This patch uses dr_alignment to calculate the (byte) alignment
instead, just like we do when creating vector MEM_REFs.

gcc/
	PR tree-optimization/94994
	* tree-vect-data-refs.c (vect_vfa_align): Use dr_alignment.

gcc/testsuite/
	PR tree-optimization/94994
	* gcc.dg/vect/pr94994.c: New test.