S/390: Fix PR94666

The vector popcount expanders use a hardcoded subreg.  This might lead
to double subregs being generated which then fail to match.  With this
patch simplify_gen_subreg is used instead to fold the subregs.

gcc/ChangeLog:

2020-04-20  Andreas Krebbel  <krebbel@linux.ibm.com>

	* config/s390/vector.md ("popcountv8hi2_vx", "popcountv4si2_vx")
	("popcountv2di2_vx"): Use simplify_gen_subreg.

gcc/testsuite/ChangeLog:

2020-04-20  Andreas Krebbel  <krebbel@linux.ibm.com>

	* g++.dg/pr94666.C: New test.