PR111048: Set arg_npatterns correctly.

PR111048: Set arg_npatterns correctly.

In valid_mask_for_fold_vec_perm_cst we set arg_npatterns always
to VECTOR_CST_NPATTERNS (arg0) because of (q1 & 0) == 0:

     /* Ensure that the stepped sequence always selects from the same
         input pattern.  */
      unsigned arg_npatterns
        = ((q1 & 0) == 0) ? VECTOR_CST_NPATTERNS (arg0)
                          : VECTOR_CST_NPATTERNS (arg1);

resulting in wrong code-gen issues.
The patch fixes this by changing the condition to (q1 & 1) == 0.

gcc/ChangeLog:
	PR tree-optimization/111048
	* fold-const.cc (valid_mask_for_fold_vec_perm_cst_p): Set arg_npatterns
	correctly.
	(fold_vec_perm_cst): Remove workaround and again call
	valid_mask_fold_vec_perm_cst_p for both VLS and VLA vectors.
	(test_fold_vec_perm_cst::test_nunits_min_4): Add test-case.