In particular, ensure that the semantic name is not NULL. This is necessary to
avoid a crash when a later pass using vsir_signature_find_element_by_name(),
e.g. vsir_program_insert_vertex_fog(), iterates over the signature including the
new element.
We cannot be redefining struct types in this rule, only referring to already
defined types. Struct type definition is handled by named_struct_spec, which
complains if the type was defined at all, regardless of class.
This is motivated by SampleId, whose presence or absence determines whether a
fragment shader runs at sample frequency or not.
In HLSL, if SV_SampleIndex is declared but not used, this results in a signature
entry, but no dcl instruction (and a zero used mask in the signature entry).
Whether the shader will actually run at sample frequency is inconsistent. NVidia
does, AMD does not, and WARP does for d3d12 but not for d3d11.
Previously vkd3d-shader relied on the dcl instruction, thereby aligning with
AMD. This was changed by 66cb2815f0. This commit
restores the previous behaviour.
sm6_metadata_get_float_value() already calls sm6_value_is_constant()
itself, and some compilers (rightly) emit a warning that the conversion
of UINT_MAX to float changes the value.
