[dxil2spv] SPIR-V validation and related fixes

[sudonatalie]

Add SPIR-V validation after the module is generated, and apply some
fixes required for validation to succeed with a passthrough pixel
shader:

• Add execution mode
• Decorate with SPIR-V Locations
• Refactor some LowerTypeVisitor logic to a visitor that is shared with
  dxil2spv
• Remove extra pointerness on stage IO vars now that it is handled by
  the above pass

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✅ Build DirectXShaderCompiler 1.0.1367 completed (commit e3fbad0b91 by @sudonatalie)

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✅ Build DirectXShaderCompiler 1.0.1368 completed (commit 7a4c88cf0a by @sudonatalie)

[jaebaek]

I am worried that readers cannot expect bool LowerTypeVisitor::visitInstruction(SpirvInstruction *instr) uses SpirvTypeVisitor object as a local variable.

I'd like to place this in std::vector<uint32_t> SpirvBuilder::takeModule() right after

LowerTypeVisitor lowerTypeVisitor(*astContext, context, spirvOptions);
mod->invokeVisitor(&lowerTypeVisitor);
// here

Is there any issue in that change?

[sudonatalie]

This was my original plan and there shouldn't be an issue with this, but after enough time trying to detangle them I decided to go this route for some middle ground between avoiding code duplication, and going overkill on a refactoring that isn't the highest priority one for us right now. I've added a clarifying comment since I think this may be easier/more beneficial to refactor later on.

[jaebaek]

I am still concerned about this because we have an implicit rule to call the visitors only in the takeModule() method of SpirvBuilder. I know it can degrade the performance because it will visit all the instructions, but doing it in the takeModule() will be better for the long term maintenance.

I also took some time to understand this part. I believe later more readers would have the same issue.

I am sorry if I bother you but I'd like to update it

LowerTypeVisitor lowerTypeVisitor(*astContext, context, spirvOptions);
mod->invokeVisitor(&lowerTypeVisitor);
SpirvTypeVisitor spvTypeVisitor(context, spirvOptions);
mod->invokeVisitor(&spvTypeVisitor);

(if it does not have technical issue).

[sudonatalie]

As discussed offline, this change doesn't work in the current state, so I've moved it to a util function as requested.

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✅ Build DirectXShaderCompiler 1.0.1383 completed (commit 7d34f1d297 by @sudonatalie)

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❌ Build DirectXShaderCompiler 1.0.1384 failed (commit 4803a54d6c by @sudonatalie)

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✅ Build DirectXShaderCompiler 1.0.1385 completed (commit ae78b893ba by @sudonatalie)