[llvm] [RegisterCoalescer] Deferring deletion of instructions in `ErasedInstrs` until the end of an iteration (PR #79820)

[Quentin Dian via llvm-commits]

DianQK wrote:

> > > > Also can you add the test from #71178 and #73519
> > > 
> > > 
> > > Hmm, this PR doesn't fix it. This test case isn't stable, and I can't test it with RegisterCoalescer.
> > 
> > 
> > I think it's a similar but different issue.
> 
> I think it's the same; it's from relying on the delete inst pointers from removePartialRedundancy

I found out why. It's a very specific case.
We erased an instruction in `coalesceLocals` and then created a new instruction because it was the same address, and we successfully deleted the record at `ErasedInstrs`. After that, we erased the new instruction again.
Finally, we will lose this record in `while (copyCoalesceWorkList(WorkList))` since `CurrentErasedInstrs` only records one iteration.



https://github.com/llvm/llvm-project/pull/79820