S/390: Fix zvector vec_double builtin

S/390: Fix zvector vec_double builtin

The (unsigned) long int to double vector conversion instructions
expect 2 immediate parameters. One for the inexact suppression
control and another one for the rounding mode.  However, the
vec_double builtin has just the vector source operand.  The 2
addtional operands need to be added with an intermediate expander.
The expanders were already there but unfortunately not wired up
correctly to the builtin.

gcc/ChangeLog:

2018-12-13  Andreas Krebbel  <krebbel@linux.ibm.com>

	* config/s390/s390-builtins.def (s390_vec_double_s64): Map to
	s390_vec_double_s64 instead of s390_vcdgb.
	(s390_vec_double_u64): Map to s390_vec_double_u64 instead of
	s390_vcdlgb.

gcc/testsuite/ChangeLog:

2018-12-13  Andreas Krebbel  <krebbel@linux.ibm.com>

	* gcc.target/s390/zvector/vec-double-1.c: New test.
	* gcc.target/s390/zvector/vec-double-2.c: New test.

From-SVN: r267084