DCE shouldn't remove unused descriptors

[expenses]

I'm performing reflection on generated SPIR-V modules to create descriptor set layouts. Unfortunately, this is a little fiddly, because whenever I comment out some code to debug something, if it means that a descriptor is no longer used, this descriptor will not be output in the final module and the generated descriptor set layouts will be wrong.

In my opinion, this sort of Dead Code Elimination should not be performed, especially with SpirvMetadata::NameVariables or NameVariables::Full. Given that SpirvMetadata::NameVariables has the comment 'Only include OpNames for public interface variables (uniforms and the like), to allow shader reflection.', I think we can say that this DCE is unwanted behaviour.

[khyperia]

We already looked into this quite a bit in #722, and tl;dr, we cannot fix this for SPIR-V versions <=1.3. Use >=1.4 if keeping unused entry parameters is needed.

[expenses]

Oh, nice! That should probably be documented somewhere.

[expenses]

Actually the DCE still seems to happen even when building for spirv-unknown-spv1.5, at least on da7c3ed.

[khyperia]

Ah, that would be a spirv-opt issue then, it's removing it - this doesn't include samp, but removing the spirv-opt invocation makes it appear: https://shader-playground.timjones.io/05a24c0565ef5fd9b8dc51457c8ad135

[expenses]

Ah, right! Looks like spirv_tools::opt::Options::preserve_bindings should be set to true then.