S/390: Improve support of 128 bit vectors in GPRs

S/390: Improve support of 128 bit vectors in GPRs

This patch improves the handling of 128 bit vectors residing in GPRs
by adding more alternatives to the move pattern.

Regression tested on s390x.

gcc/ChangeLog:

2017-03-24  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* config/s390/constraints.md: Add comments.
	(jKK): Reject element sizes > 8 bytes.
	* config/s390/s390.c (s390_split_ok_p): Enable splitting also for
	s_operands.
	* config/s390/s390.md: Add the s_operand checks formerly in
	s390_split_ok_p to various splitters where they are still
	required.
	* config/s390/vector.md ("mov<mode>" V_128): Add GPR alternatives
	for 128 bit vectors.  Plus two splitters.

From-SVN: r246445