[spirv] Handle invalid arg for residency code.

[ehsannas]

Fixes #3045

[AppVeyorBot]

✅ Build DirectXShaderCompiler 1.0.3495 completed (commit efbb789351 by @ehsannas)

[jaebaek]

What happens if residencyCode is nullptr?
Why don't we change this condition to if (residencyCode == nullptr || residencyCode->isRValue()) {?

[ehsannas]

this function (SpirvEmitter::processBufferTextureLoad) handles the Load methods. There are 2 Load methods that are defined for textures and buffer: one method that just does the load, and one method that does the load and also returns the status of the operation.

For example, for Texture2D we have these 2 methods: (spec)

Load(int,int) : Reads texture data.
Load(int,int,uint) : Reads texture data and returns status of the operation.

In the first method, residencyCode is in fact nullptr and it should not be an error.

[jaebaek]

Got it. It would be better to add a comment to briefly explain it.

[AppVeyorBot]

✅ Build DirectXShaderCompiler 1.0.3536 completed (commit 9377076ef5 by @ehsannas)