s390: Try to emit vlbr/vstbr instead of vperm et al.

s390: Try to emit vlbr/vstbr instead of vperm et al.

gcc/ChangeLog:

	* config/s390/s390.cc (expand_perm_as_a_vlbr_vstbr_candidate):
	New function which handles bswap patterns for vec_perm_const.
	(vectorize_vec_perm_const_1): Call new function.
	* config/s390/vector.md (*bswap<mode>): Fix operands in output
	template.
	(*vstbr<mode>): New insn.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/s390.exp: Add subdirectory vxe2.
	* gcc.target/s390/vxe2/vlbr-1.c: New test.
	* gcc.target/s390/vxe2/vstbr-1.c: New test.
	* gcc.target/s390/vxe2/vstbr-2.c: New test.