tree-optimization/96376 - do not check alignment for invariant loads

The testcases show that we fail to disregard alignment for invariant
loads.  The patch handles them like we handle gather and scatter.

2021-01-15  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96376
	* tree-vect-stmts.c (get_load_store_type): Disregard alignment
	for VMAT_INVARIANT.