[HLSL][SPIRV] Generate sign extended image instructions

[s-perron]

In HLSL, a RWBuffer is suppose to be loading value as if they are signed integers. This is important because if the format of the image is 8-bit, then the value on a load is implicitly sign extended. The problem in the current Clang implementation is that the signedness of the sampled type is lost because llvm-ir interpreted whether or not an integer is sign or not based on the instruction that is using it.

Without knowing the signedness of the sample type, SPIR-V generates all code assuming the integers are unsigned. This causes problems.

There are two possible solutions in the SPIR-V:

1. Pass information along so that the sampled type can be modified to be a signed integer.
2. Pass information along so that the OpImage* instructions can get the signExtend image operand when necessary.

The difficulty with 1 is that the sampled type of the image must match the Texel components of the OpImage* instructions. This means that the type information will have to propagate further.

Seems like 2 is the best solution. It is a more local change, making the code generation easier.

Now we still need to figure out how to pass the information from the FE to the BE. There are again a few options

1. Use a new target type that has a bit for isSigned as is done for DXIL.
2. Incorporate the sign into the type for the spirv.Image type.
3. Add a target attribute to the "gethandlefrombinding" call.
4. Add an annotation to the loads and stores.

Option 4 does not work. The annotation would have to be metadata, which the optimizer is allowed to drop.

Option 2 does not work well either. It would require special handling to make sure the sampled type matches other integers. Not a reasonable option.

To know if 1 will work, I'll have to see if the SPIR-V backend has easy access to the lllvm type when generating the load and stores. The SPIR-V type will have lost the signedness informations.

If that does not work, we will have to go with option 3. However, that has it own uglyness.