[SPIRV][Vulkan] OpCapability Kernel being set on Shaders

[llvm-beanz]

I'm seeing a few cases where Shaders are being generated with OpCapability Kernel applied in addition to OpCapability Shader. Seems like those two should be mutually exclusive.

Reproduction links:

• https://godbolt.org/z/Mx8Ef17e5
• https://godbolt.org/z/3Ez1eY1qj
• https://godbolt.org/z/6EY9EaPGv

[s-perron]

It is being added because the FPFastMathMode decoration requires it. That decoration should not be added for shaders.

[s-perron]

For shaders, the decoration is available when using FloatControl2. This is not a required feature in Vulkan yet, and is not widely available. I suggest that for now, we simply avoid adding the decoration for shaders.

[s-perron]

I have a PR that will avoid creating invalid SPIR-V. However, it does not properly handle all cases. A more careful design will be required for handling HLSL floating point.

• When FloatControls2 is available, we need to set the appropriate default fast math mode avoiding API specificed defaults.
• When it is not available, the a different set of execution modes and decorations are used. They need to be added appropriately.