PR target/68483
	* tree-vect-generic.c (lower_vec_perm): If VEC_PERM_EXPR
	is valid vec_shr pattern, don't lower it even if can_vec_perm_p
	returns false.
	* optabs.c (shift_amt_for_vec_perm_mask): Return NULL_RTX
	whenever first is nelt or above.  Don't mask expected with
	2 * nelt - 1.

	* gcc.target/i386/pr68483-1.c: New test.
	* gcc.target/i386/pr68483-2.c: New test.

From-SVN: r230797