i386: Fix zero/sign extend expanders [PR95229]

2020-05-20  Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog:
	PR target/95229
	* config/i386/sse.md (<code>v8qiv8hi2): Use
	simplify_gen_subreg instead of simplify_subreg.
	(<code>v8qiv8si2): Ditto.
	(<code>v4qiv4si2): Ditto.
	(<code>v4hiv4si2): Ditto.
	(<code>v8qiv8di2): Ditto.
	(<code>v4qiv4di2): Ditto.
	(<code>v2qiv2di2): Ditto.
	(<code>v4hiv4di2): Ditto.
	(<code>v2hiv2di2): Ditto.
	(<code>v2siv2di2): Ditto.

gcc/testsuite/ChangeLog:
	PR target/95229
	* g++.target/i386/pr95229.C: New test.