[llvm] [MachineScheduler] Fix physreg dependencies of ExitSU (PR #123541)

[Jay Foad via llvm-commits]

jayfoad wrote:

Seems reasonable to me, both the code changes and the AMDGPU diffs.

I wonder if it might be possible to let addPhysRegDeps handle this instead of reimplementing some of that logic in addSchedBarrierDeps. E.g. maybe the "Walk the list of instructions" loop in buildSchedGraph could start by processing the use operands of RegionEnd (aka ExitSU/ExitMI), and end by processing the def operands of RegionBegin.

https://github.com/llvm/llvm-project/pull/123541