i386: Optimize _mm_unpacklo_epi8 of 0 vector as second argument or similar VEC_PERM_E...

i386: Optimize _mm_unpacklo_epi8 of 0 vector as second argument or similar VEC_PERM_EXPRs into pmovzx [PR95905]

The following patch adds patterns (so far 128-bit only) for permutations
like { 0 16 1 17 2 18 3 19 4 20 5 21 6 22 7 23 } where the second
operand is CONST0_RTX CONST_VECTOR to be emitted as pmovzx.

2021-01-13  Jakub Jelinek  <jakub@redhat.com>

	PR target/95905
	* config/i386/predicates.md (pmovzx_parallel): New predicate.
	* config/i386/sse.md (*sse4_1_zero_extendv8qiv8hi2_3): New
	define_insn_and_split pattern.
	(*sse4_1_zero_extendv4hiv4si2_3): Likewise.
	(*sse4_1_zero_extendv2siv2di2_3): Likewise.

	* gcc.target/i386/pr95905-1.c: New test.
	* gcc.target/i386/pr95905-2.c: New test.