[llvm] [PHIElimination] Handle subranges in LiveInterval updates (PR #69429)

[Matthias Braun via llvm-commits]

https://github.com/MatzeB commented:

* Looks mostly fine
* I have some questions on one `if` condition below.
* Should probably mention in the summary that this adds subrange liveness tracking for critical edge splitting and phi elimination.

https://github.com/llvm/llvm-project/pull/69429