[AMDGPU] Simplify GFX11/GFX12 FLAT instruction definitions. NFC. (#85819)

- Give the tablegen record for the Real the same name as the tablegen
  record for the pseudo. This removes all cases where the same
  instruction name has to be mentioned more than once on the definition
  line.
- Use multiclasses for all Real definitions, to allow suffixes to be
  added bit by bit, e.g. first _SADDR and then _gfx11.

This is a similar approach to the one used in BUFInstructions.td.