[SPIR-V] Fix mesh shader output reads with SPV_NV_mesh_shader #9035

Description
GL_NV_mesh_shader  lets a mesh shader read back its own outputs, but DXC's SPIR-V back-end handles HLSL reads of mesh outputs badly under  SPV_NV_mesh_shader :

• Reading a whole output element, such as  verts[2] = verts[1]; , is rejected by a Sema error.
• Reading a member or component, such as  verts[0].pos , is accepted but compiles to  OpUndef  instead of a load from the Output variable. The shader silently computes garbage.

Steps to Reproduce

struct VertOut {
  float4 pos : SV_Position;
  float3 color : COLOR0;
};

[outputtopology("triangle")]
[numthreads(1, 1, 1)]
void main(out vertices VertOut verts[3], out indices uint3 tris[1]) {
  SetMeshOutputCounts(3, 1);
  verts[0].pos = float4(0, 0, 0, 1);
  verts[0].color = float3(1, 0, 0);
  verts[1].pos = verts[0].pos + float4(1, 0, 0, 0);
  verts[1].color = verts[0].color.zyx;
  verts[2] = verts[1];
  tris[0] = uint3(0, 1, 2);
}

 dxc -T ms_6_5 -E main -spirv -fspv-target-env=vulkan1.3 -fspv-extension=SPV_NV_mesh_shader t.hlsl

Actual Behavior

t.hlsl:14:14: error: output arrays of a mesh shader can not be read from
  verts[2] = verts[1];
             ^

With line 14 removed, compilation succeeds, but the member reads become  OpUndef :

%34 = OpUndef %v4float
%35 = OpUndef %v3float
%39 = OpFAdd %v4float %34 %17
%41 = OpVectorShuffle %v3float %35 %35 2 1 0

Expected: each read loads from  gl_Position  /  out_var_COLOR0  at the given index.

Environment

• DXC version 1.10.2609.0
• Host Operating System Windows