[libclc] [libclc] Bring back fma for clspv (PR #180693)

[Romaric Jodin via cfe-commits]

rjodinchr wrote:

> Am I understanding correct that we can use @llvm.fma for SPIRV target once https://github.com/google/clspv/issues/1533 is done?

My understanding is that once https://github.com/google/clspv/issues/1533, devices supporting [VK_KHR_shader_fma](https://docs.vulkan.org/refpages/latest/refpages/source/VK_KHR_shader_fma.html) would be able to use `@llvm.fma` through `SPV_KHR_fma`.

But the fact that `clspv` supports `SPV_KHR_fma` does not mean that the device supports it. Thus we will need to be able to handle both paths forever I would say

@alanbaker @dneto, do you confirm?

https://github.com/llvm/llvm-project/pull/180693