This adds support for the vector load element and zero instruction and
makes sure it is used when initializing vectors with elements while
setting the rest to 0.

gcc/ChangeLog:

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

	* config/s390/s390.c (s390_expand_vec_init): Use vllezl
	instruction if possible.
	* config/s390/vector.md (vec_halfnumelts): New mode
	attribute.
	("*vec_vllezlf<mode>"): New pattern.

gcc/testsuite/ChangeLog:

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

	* gcc.target/s390/vxe/vllezlf-1.c: New test.

From-SVN: r246455