PR target/107548: Handle vec_select in STV on x86.

PR target/107548: Handle vec_select in STV on x86.

This patch enhances x86's STV pass to handle VEC_SELECT during general
scalar chain conversion, performing SImode scalar extraction from V4SI
and DImode scalar extraction from V2DI in vector registers.

The motivating test case from bugzilla is:

typedef unsigned int v4si __attribute__((vector_size(16)));

unsigned int f (v4si a, v4si b)
{
  a[0] += b[0];
  return a[0] + a[1];
}

currently with -O2 -march=znver2 this generates:

	vpextrd	$1, %xmm0, %edx
	vmovd	%xmm0, %eax
	addl	%edx, %eax
	vmovd	%xmm1, %edx
	addl	%edx, %eax
	ret

which performs three transfers from the vector unit to the scalar unit,
and performs the two additions there.  With this patch, we now generate:

	vmovdqa	%xmm0, %xmm2
	vpshufd	$85, %xmm0, %xmm0
	vpaddd	%xmm0, %xmm2, %xmm0
	vpaddd	%xmm1, %xmm0, %xmm0
	vmovd	%xmm0, %eax
	ret

which performs the two additions in the vector unit, and then transfers
the result to the scalar unit.  Technically the (cheap) movdqa isn't
needed with better register allocation (or this could be cleaned up
during peephole2), but even so this transform is still a win.

2022-12-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/107548
	* config/i386/i386-features.cc (scalar_chain::add_insn): The
	operands of a VEC_SELECT don't need to added to the scalar chain.
	(general_scalar_chain::compute_convert_gain) <case VEC_SELECT>:
	Provide gains for performing STV on a VEC_SELECT.
	(general_scalar_chain::convert_insn): Convert VEC_SELECT to pshufd,
	psrldq or no-op.
	(general_scalar_to_vector_candidate_p): Handle VEC_SELECT of a
	single element from a vector register to a scalar register.

gcc/testsuite/ChangeLog
	PR target/107548
	* gcc.target/i386/pr107548-1.c: New test V4SI case.
	* gcc.target/i386/pr107548-2.c: New test V2DI case.