vkd3d-shader/hlsl: Replace loads with constants in copy prop.

If a hlsl_ir_load loads a variable whose components are stored from different
instructions, copy propagation doesn't replace it.

But if all these instructions are constants (which currently is the case
for value constructors), the load could be replaced with a constant value.
Which is expected in some other instructions, e.g. texel_offsets when
using aoffimmi modifiers.

For instance, this shader:

```
sampler s;
Texture2D t;

float4 main() : sv_target
{
    return t.Gather(s, float2(0.6, 0.6), int2(0, 0));
}
```

results in the following IR before applying the patch:
```
  float | 6.00000024e-01
  float | 6.00000024e-01
   uint | 0
        | = (<constructor-2>[@4].x @2)
   uint | 1
        | = (<constructor-2>[@6].x @3)
 float2 | <constructor-2>
    int | 0
    int | 0
   uint | 0
        | = (<constructor-5>[@11].x @9)
   uint | 1
        | = (<constructor-5>[@13].x @10)
   int2 | <constructor-5>
 float4 | gather_red(resource = t, sampler = s, coords = @8, offset = @15)
        | return
        | = (<output-sv_target0> @16)
```

and this IR afterwards:
```
 float2 | {6.00000024e-01 6.00000024e-01 }
   int2 | {0 0 }
 float4 | gather_red(resource = t, sampler = s, coords = @2, offset = @3)
        | return
        | = (<output-sv_target0> @4)
```

tests/hlsl-initializer-objects.shader_test

@@ -29,7 +29,7 @@ draw quad
 probe all rgba (0.2, 0.2, 0.2, 0.1)
 
 
-[pixel shader todo]
+[pixel shader]
 Texture2D tex;
 
 struct foo
@@ -48,11 +48,11 @@ float4 main() : sv_target
 }
 
 [test]
-todo draw quad
-todo probe all rgba (31.1, 41.1, 51.1, 61.1) 1
+draw quad
+probe all rgba (31.1, 41.1, 51.1, 61.1) 1
 
 
-[pixel shader todo]
+[pixel shader]
 Texture2D tex1;
 Texture2D tex2;