tree-optimization/111048 - avoid flawed logic in fold_vec_perm

The following avoids running into somehow flawed logic in fold_vec_perm
for non-VLA vectors.

	PR tree-optimization/111048
	* fold-const.cc (fold_vec_perm_cst): Check for non-VLA
	vectors first.

	* gcc.dg/torture/pr111048.c: New testcase.