[llvm] [SPIR-V] Allow FPFastMathMode decoration on OpFmaKHR (PR #206143)

[Juan Manuel Martinez Caamaño via llvm-commits]

https://github.com/jmmartinez commented:

>From reading [SPV_KHR_fma](https://github.khronos.org/SPIRV-Registry/extensions/KHR/SPV_KHR_fma.html) and [FP_Fast_Math_Mode](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#FP_Fast_Math_Mode) it's not clear to me that `OpFMAKHR` allows for a fast-math-flags decoration (I'm not saying it shouldn't).

Should we try to correct that in the spec?

https://github.com/llvm/llvm-project/pull/206143