Fix unused shared memory failing validation

[Firestar99]

This fixes validation errors caused by unused shared memory and enables it to be DCE-ed away.

Params of entry point interfaces will now be DCE-ed by rust-gpu as well, previously only spirv-opt does some DCE on them but it leaves unused shared memory params as-is. As with spirv-opt, SpirvBuilder.optimizer.preserve_bindings = true will disable any DCE of unused entry point params.

On a vulkan1.1/spv1.3 or lower preserve_bindings does not work on any param that is not of class input or output, so shared memory will be DCE-ed anyway, due to there being no references to those instructions. Whereas vulkan1.2+/spv1.4+ added the requirement for all OpVariables to be mentioned in the entry point instruction, allowing us to preserve them.

close #455

[Firestar99]

@eddyb requesting rereview for the 2 new commits. I noticed that on vulkan1.1 we did DCE the unused shared memory, so investigated a bit and found a way to DCE it on other targets as well (see desc).