[llvm] [AMDGPU] Replace tablegen field is_gfx940_xdl with is_sgemm. NFC. (PR #143865)

[Jay Foad via llvm-commits]

jayfoad wrote:

I'm not sure if this patch makes sense. I'd really appreciate any comments.

My understanding is:
- before gfx942, "XDL" means MFMA_*_F16/BF16/F32/I8 (i.e. everything except F64)
- from gfx942, "XDL" means MFMA_*_F16/BF16/XF32/I8 (i.e. everything except F32 and F64)

But maybe this is not accurate? The way `is_sgemm` is defined does not seem to match this description, e.g. some `_f32` instructions are included but other ones are not.

https://github.com/llvm/llvm-project/pull/143865