S/390: Add support for vec_shr

S/390: Add support for vec_shr

gcc/ChangeLog:

2017-09-26  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* config/s390/predicates.md ("const_shift_by_byte_operand"): New
	predicate.
	* config/s390/vector.md ("*vec_srb<mode>"): Change modes to V_128
	and V16QI.
	("*vec_slb<mode>"): New insn pattern.
	("vec_shr_<mode>"): New expander.
	* config/s390/vx-builtins.md ("vec_slb<mode>"): Turn into expander
	and force the shift count operand to V16QImode.
	("vec_srb<mode>"): Set shift count mode to V16QI.

From-SVN: r253193