Implement extension SPV_KHR_float_controls2 (#3475)
First attempt at implementing
[SPV_KHR_float_controls2](https://github.khronos.org/SPIRV-Registry/extensions/KHR/SPV_KHR_float_controls2.html).
- When doing SPIRV->LLVM-IR, we first read the
`ExecutionModeFPFastMathDefault` for every kernel, and if instructions
in that kernel do not specify a particular `FPFastMathMode`, we use the
kernel one.
- `ExecutionModeFPFastMathDefault` is **not** propagated down to the
callees.
- According to
[SPV_KHR_float_controls2#issues](https://github.khronos.org/SPIRV-Registry/extensions/KHR/SPV_KHR_float_controls2.html#_issues);
we do not have an equivalent of LLVM's `afn` flag. If we map `fadd fast
float %a, %b` to SPIRV and back, it becomes `fadd reassoc nnan ninf nsz
arcp contract float %a, %b` losing the `afn` flag.
- `ContractionOff` and `SignedZeroInfNanPreserve` are translated to a
`ExecutionModeFPFastMathDefault` with all flags set to 0.
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/8dfdbae11aaf8ba
