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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I think this is an easier to understand version of the fix. The test can be cleaned up a bit though.

Can you drop the IR section, delete the registers section, use -run-pass=none -simplify-mir to clean up most of the unnecessary bits? 

Also can you add the test from #71178 and #73519

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