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

[Quentin Colombet via llvm-commits]

qcolombet wrote:

> > This cleans things up a bit and _more accurately models subreg lane liveness_
> 
> This suggests that it is not NFC? Is there any way of testing this?

I think you're right that the wording suggests that, but the "proof" that @jrbyrnes wrote in the commit message shows that this is NFC.
I haven't checked yet if the assumption that `LiveAfter` is `0` is true though.

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