[AMDGPU] Use BUF multiclasses to reduce repetition. NFC. (#84003)

Define BUF Real instructions with this general pattern for all
architectures (not just GFX11):

  multiclass Something_Real_gfx11<...> {
    defvar ps = !cast<Pseudo>(NAME);
    def _gfx11 : ...;
  }

This allows removing a huge amount of repetition in the definitions of
individual Real instructions, where they would have to !cast their own
name to a Pseudo and pass that in as a class argument.

Author: jayfoad