i386: Improve avx* vector concatenation [PR93594]

i386: Improve avx* vector concatenation [PR93594]

The following testcase shows that for _mm256_set*_m128i and similar
intrinsics, we sometimes generate bad code.  All 4 routines are expressing
the same thing, a 128-bit vector zero padded to 256-bit vector, but only the
3rd one actually emits the desired vmovdqa      %xmm0, %xmm0 insn, the
others vpxor    %xmm1, %xmm1, %xmm1; vinserti128        $0x1, %xmm1, %ymm0, %ymm0
The problem is that the cast builtins use UNSPEC_CAST which is after reload
simplified using a splitter, but during combine it prevents optimizations.
We do have avx_vec_concat* patterns that generate efficient code, both for
this low part + zero concatenation special case and for other cases too, so
the following define_insn_and_split just recognizes avx_vec_concat made of a
low half of a cast and some other reg.

2020-02-06  Jakub Jelinek  <jakub@redhat.com>

	PR target/93594
	* config/i386/predicates.md (avx_identity_operand): New predicate.
	* config/i386/sse.md (*avx_vec_concat<mode>_1): New
	define_insn_and_split.

	* gcc.target/i386/avx2-pr93594.c: New test.