[llvm] [ConstraintElim] decompose subtract with guard as precondition (PR #209615)

[Mikael Holmen via llvm-commits]

mikaelholmen wrote:

Hi @AbhayKanhere ,

I noticed that this patch makes constraint-elimination significantly slower on some input.
An example:
```
opt -passes="constraint-elimination" bbi-120619.ll -o /dev/null
```
Before the patch it terminated pretty much instantly. With the patch it takes several minutes.

The bbi-120619.ll input looks like a sort of degenerated case with a list of several "sub" and I've only seen this show up with a non-standard opt pipeline, but maybe it's something to improve anyway?

[bbi-120619.ll.gz](https://github.com/user-attachments/files/31905814/bbi-120619.ll.gz)


https://github.com/llvm/llvm-project/pull/209615