Erroneous warning about 0 iteration loops with specialization constants in GLSL compatibility mode

[juliusikkala]

When the loop length is dependent on a specialization constant that is initialized to zero, slangc reports that the "loop runs for 0 iterations and will be removed. It shouldn't, since the value of the specialization constant isn't actually known at compile time. Luckily, even with -O2, it doesn't actually appear to get removed, judging from -target glsl output still emitting the loop anyway. So it's probably just the diagnostics.

Source for replicating:

// test.comp
#version 460
layout(constant_id = 0) const uint A = 0;
layout(local_size_x = 1) in;

layout(binding = 0) buffer output_buffer
{
    float output[];
};

void main()
{
    for(uint i = 0; i < A; ++i)
    {
        output[i] = i;
    }
}

(The output buffer is only there to prevent the compiler from optimizing the loop away, it is not particularly necessary to replicate this bug.)

Output:

test.comp(13): warning 30505: the loop runs for 0 iterations and will be removed.
    for(uint i = 0; i < A; ++i)
    ^~~

[juliusikkala]

It appears that this is incredibly easy to work around; just not using the const keyword with the specialization constant is enough. Since basically all GLSL shaders with specialization constants use the const keyword, I'll submit a PR to fix this anyway.