[llvm] [AMDGPU] Check MIR after SIMemoryLegalizer instead of final ISA (PR #90601)

[Jay Foad via llvm-commits]

jayfoad wrote:

> > > I am not a fan of the idea to convert all memory legalizer tests to mir. Maybe you can add some specific mir tests instead?
> > 
> > 
> > Well all of `memory-legalizer-fence.ll` seems pretty useless at the moment since it doesn't show any of the waitcnts that were inserted. (OK it does still show cache invalidate instructions.)
> > As an alternative, how about changing SIInsertWaitcnts so that it does not remove redundant waitcnts at `-O0` and using that for all the memory legalizer tests?
> 
> I like this much more: we will see what is produced without decoding and it shall work faster too. Now these tests are very slow.

#90716

https://github.com/llvm/llvm-project/pull/90601