[llvm] change contents of ScalarEvolution from private to protected (PR #83052)

[Joshua Ferguson via llvm-commits]

skewballfox wrote:

> you're a first time LLVM contributor (who was trying to use Enzyme on Windows and found this was able to help remedy and made this PR),

I'm actually on fedora and someone mentioned this as a good place to start contributing to LLVM coming from rust.

> a much bigger lift, but would you be able to add the optional must-exit functionality into SE itself?

I can do that, or at least attempt to. Noob question, how should I go about testing changes to ScalarEvolution? I'm guessing there are some relevant test under enzyme I should migrate with that as well? 

https://github.com/llvm/llvm-project/pull/83052