[HLSL][SPRIV] Handle signed RWBuffer correctly (#144774)

In Vulkan, the signedness of the accesses to images has to match the
signedness of the backing image.
    
See

https://docs.vulkan.org/spec/latest/chapters/textures.html#textures-input,
where it says the behaviour is undefined if
    
> the signedness of any read or sample operation does not match the
signedness of the image’s format.
    
Users who define say an `RWBuffer<int>` will create a Vulkan image with
a signed integer format. So the HLSL that is generated must match that
expecation.
    
The solution we use is to generate a `spirv.SignedImage` target type for
signed integer instead of `spirv.Image`. The two types are otherwise the
same.
    
The backend will add the `signExtend` image operand to access to the
image to ensure the image is access as a signed image.
    
Fixes #144580

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