[clang] [AArch64][SME2] Refactor arm_sme.td into multiclasses (PR #78169)

[Sander de Smalen via cfe-commits]

https://github.com/sdesmalen-arm requested changes to this pull request.

There isn't that much commonality factored out by these multi-classes as I had initially hoped, but I can understand why it's difficult to group these together in a better way. 

Given the above and the fact that the original format is a bit easier to search in (i.e. I can just search for `svmls.*za32.*vg1x4` and I would find the intrinsic that I'm looking for), I think I'd prefer to stick with the original format.

https://github.com/llvm/llvm-project/pull/78169