i386: Refactor V2DI arithmetic right shift expansion for STV.

This patch refactors ashrv2di RTL expansion into a function so that it may
be reused by a pre-reload splitter, such that DImode right shifts may be
considered candidates during the Scalar-To-Vector (STV) pass.  Currently
DImode arithmetic right shifts are not considered potential candidates
during STV, so for the following testcase:

long long m;
typedef long long v2di __attribute__((vector_size (16)));
void foo(v2di x) { m = x[0]>>63; }

We currently see the following warning/error during STV2
>  r101 use in insn 7 isn't convertible

And end up generating scalar code with an interunit move:

foo:	movq    %xmm0, %rax
        sarq    $63, %rax
        movq    %rax, m(%rip)
        ret

With this patch, we can reuse the RTL expansion logic and produce:

foo:	psrad   $31, %xmm0
        pshufd  $245, %xmm0, %xmm0
        movq    %xmm0, m(%rip)
        ret

Or with the addition of -mavx2, the equivalent:

foo:    vpxor   %xmm1, %xmm1, %xmm1
        vpcmpgtq        %xmm0, %xmm1, %xmm0
        vmovq   %xmm0, m(%rip)
        ret

The only design decision of note is the choice to continue lowering V2DI
into vector sequences during RTL expansion, to enable combine to optimize
things if possible.  Using just define_insn_and_split potentially misses
optimizations, such as reusing the zero vector produced by vpxor above.
It may be necessary to tweak STV's compute gain at some point, but this
patch controls what's possible (rather than what's beneficial).

2024-08-06  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386-expand.cc (ix86_expand_v2di_ashiftrt): New
	function refactored from define_expand ashrv2di3.
	* config/i386/i386-features.cc (general_scalar_to_vector_candidate_p)
	<case ASHIFTRT>: Handle like other shifts and rotates.
	* config/i386/i386-protos.h (ix86_expand_v2di_ashiftrt): Prototype.
	* config/i386/sse.md (ashrv2di3): Call ix86_expand_v2di_ashiftrt.
	(*ashrv2di3): New define_insn_and_split to enable creation by stv2
	pass, and splitting during split1 reusing ix86_expand_v2di_ashiftrt.

gcc/testsuite/ChangeLog
	* gcc.target/i386/sse2-stv-2.c: New test case.