[llvm] [NFC] Refactor looping over recomputeLiveIns into function (PR #88040)

[Kai Nacke via llvm-commits]

redstar wrote:

> > LGTM. Perhaps it's worth checking CTMark times, I'd improvement (perhaps not measurable overall), but just to avoid surprises.
> 
> Good point. I'll try a run.

Sorry for the delay, I ran into some trouble with CTMark. Turned out that the commit I based my changes on had a problem.
I see a small degradation in compile time performance, which surprises me since I also checked some of the test cases, and the number of calls to `recomputeLiveIns()` decreased. I like to understand this first before committing the change. 

https://github.com/llvm/llvm-project/pull/88040