i386: Split SUBREGs of SSE vector registers into vec_select insns.

i386: Split SUBREGs of SSE vector registers into vec_select insns.

This patch is the final piece in the series to improve the ABI issues
affecting PR 88873.  The previous patches tackled inserting DFmode
values into V2DFmode registers, by introducing insvti_{low,high}part
patterns.  This patch improves the extraction of DFmode values from
V2DFmode registers via TImode intermediates.

I'd initially thought this would require new extvti_{low,high}part
patterns to be defined, but all that's required is to recognize that
the SUBREG idioms produced by combine are equivalent to (forms of)
vec_select patterns.  The target-independent middle-end can't be sure
that the appropriate vec_select instruction exists on the target,
hence doesn't canonicalize a SUBREG of a vector mode as a vec_select,
but the backend can provide a define_split stating where and when
this is useful, for example, considering whether the operand is in
memory, or whether !TARGET_SSE_MATH and the destination is i387.

For pr88873.c, gcc -O2 -march=cascadelake currently generates:

foo:    vpunpcklqdq     %xmm3, %xmm2, %xmm7
        vpunpcklqdq     %xmm1, %xmm0, %xmm6
        vpunpcklqdq     %xmm5, %xmm4, %xmm2
        vmovdqa %xmm7, -24(%rsp)
        vmovdqa %xmm6, %xmm1
        movq    -16(%rsp), %rax
        vpinsrq $1, %rax, %xmm7, %xmm4
        vmovapd %xmm4, %xmm6
        vfmadd132pd     %xmm1, %xmm2, %xmm6
        vmovapd %xmm6, -24(%rsp)
        vmovsd  -16(%rsp), %xmm1
        vmovsd  -24(%rsp), %xmm0
        ret

with this patch, we now generate:

foo:	vpunpcklqdq     %xmm1, %xmm0, %xmm6
        vpunpcklqdq     %xmm3, %xmm2, %xmm7
        vpunpcklqdq     %xmm5, %xmm4, %xmm2
        vmovdqa %xmm6, %xmm1
        vfmadd132pd     %xmm7, %xmm2, %xmm1
        vmovsd  %xmm1, %xmm1, %xmm0
        vunpckhpd       %xmm1, %xmm1, %xmm1
        ret

The improvement is even more dramatic when compared to the original
29 instructions shown in comment #8.  GCC 13, for example, required
12 transfers to/from memory.

2023-08-04  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/sse.md (define_split): Convert highpart:DF extract
	from V2DFmode register into a sse2_storehpd instruction.
	(define_split): Likewise, convert lowpart:DF extract from V2DF
	register into a sse2_storelpd instruction.

gcc/testsuite/ChangeLog
	* gcc.target/i386/pr88873.c: Tweak to check for improved code.