So long as the target of the alias is predicated with HasImageInsts or
similar, the alias itself does not need this predicate.

AMDGPUMnemonicAlias is a MnemonicAlias that inherits from
GCNPredicateControl, so that we can set predicates on the alias the same
way as Instructions.
Use AssemblerPredicate instead of Requires on aliases

NFC.

Pass the Pseudo (instead of its name) into EXP_Real_Row and
EXP_Real_ComprVM since it is already available in all subclasses.

Using multiclasses for the Real instruction definitions has a couple of
benefits:
- It avoids repeating information that was already specified when
  defining the corresponding pseudo, like the row and done bits.
- It allows commoning up the Real definitions for architectures which
  are mostly the same, like GFX11 and GFX12.

In practice maybeAtomic = 0 is used to prevent SIMemoryLegalizer from
interfering with instructions that are mayLoad or mayStore but lack
MachineMemOperands. These instructions should be the exception not the
rule, so this patch sets maybeAtomic = 1 by default and only overrides
it to 0 where necessary.

In GFX12 the exp instruction is renamed to export, but exp is still
accepted as an alias.

Co-authored-by: Mateja Marjanovic <mateja.marjanovic@amd.com>

Differential Revision: https://reviews.llvm.org/D127671