vkd3d-shader/hlsl: Emit fixmes on non-constant vector addressing.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=56162

Storing to a vector component using a non-constant index is not allowed
on profiles lower than 6.0. Unless this happens inside a loop that can be
unrolled, which we are not doing yet.

For this reason, a validate_nonconstant_vector_store_derefs pass is
added to detect these cases.

Ideally we would want to emit an hlsl_error on this pass, but before
implementing loop unrolling, we could reach this point on valid HLSL.

Also, as pointed out by Nikolay in the mentioned bug, currently
new_offset_from_path_index() fails an assertion when this happens,
because it expects an hlsl_ir_constant, so a check is added.
It also felt correct to emit an hlsl_fixme there, despite the
redundancy.

tests/hlsl/vector-indexing.shader_test

@@ -49,3 +49,16 @@ float4 main() : SV_TARGET
 [test]
 draw quad
 probe all rgba (2.0, 2.0, 2.0, 2.0)
+
+
+[pixel shader fail(sm<6) todo]
+float4 v;
+float i;
+
+float4 main() : sv_target
+{
+    float4 p = v;
+    p[i] = 2.0;
+
+    return 0;
+}