[llvm] [CodeGen] Use first EHLabel as a stop gate for live range shrinking (PR #114195)

[via llvm-commits]

MuellerMP wrote:

Updated the patch again:
- Added a comment explaining the initial SkipPHIsLabelsAndDebug call for EHPads
- Added a iterator-equals-end check before the initial BuildInstOrderMap for EHPads due to BuildInstOrderMap derefing the iterator
- Now using SkipPHIsLabelsAndDebug infront and inside the loop thus eliminating the if continue

Did further testing with this patch and it seems to achieve the desired result.
I hope this clarifies my fix and also incorporates the adjustments regarding the Skip func in a meaningful way.

@arsenm would you be ok with merging this for me since i don't have write access?

https://github.com/llvm/llvm-project/pull/114195