[llvm] [AMDGPU] Introduce a pseudo mnemonic for S_DELAY_ALU in MIR. (PR #96004)

[Jay Foad via llvm-commits]

jayfoad wrote:

I think this looks pretty nice, unless there are any principled objections to using custom formatted MIR operands?

Doing a similar thing for `s_waitcnt` would make some of our MIR tests _way_ more readable.

https://github.com/llvm/llvm-project/pull/96004