vkd3d-shader/d3dbc: Fix regression when declaring structs in SM1.

This commit is part of a series of commits intended to remove all uses of hlsl_type_get_regset().

However, I think it deserves to be upstreamed sooner since it solves a rather important SM1 regression (explained below) introduced in e0031d2a .

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In SM1 we can expect all variables to always belong to a single regset. structs in particular, should always be allocated to HLSL_REGSET_NUM, since they are only allowed if all their components are numeric.

We are not covering the structs case because of the use of hlsl_type_get_regset(), which is currently not defined for structs.

So the current shader

struct
{
    float4 a;
    float4 b;
} apple;

float4 main() : sv_target
{
    return apple.a + apple.b;
}

fails with

vkd3d/libs/vkd3d-shader/hlsl.c:224: Aborting, reached unreachable code.

The solution is to iterate over all regsets to find the one where the variable is allocated (if any), and ignore all others.