[llvm] [RegisterPressure] NFC: Clean up RP handling for instructions with overlapping Def/Use (PR #109875)

[Quentin Colombet via llvm-commits]

qcolombet wrote:

> I haven't checked yet if the assumption that `LiveAfter` is `0` is true though.

I confirm that this is true. If we already counted the regunit there's no point in adding it again, hence the behavior that @jrbyrnes described.
(Note that I'm neither approving or denying the PR. @jayfoad if you feel comfortable reviewing go for it! I haven't yet; only the commit message :))

https://github.com/llvm/llvm-project/pull/109875