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

[Joshua Ferguson via llvm-commits]

skewballfox wrote:

@nikic I've finished the changes you requested. The only thing left is to test Scalar Evolution with `AssumeLoopExit` set to true. I see from the LLVM docs this is supposed to be a [regression test](https://llvm.org/docs/TestingGuide.html#regression-tests)

I'm still working out the details of how this should be done. Here's my very fuzzy understanding of what needs to happen
- I either generate new llvmIR code (how wsmoses did using godbolt, or reuse the existing ir code under `llvm/test/Analysis/ScalarEvolution`  to capture the baseline behavior
- I add an ifdef macro to set assumeLoopExist to true when instantiation SE, similar to how `EXPENSIVE_CHECKS` works. I have that locally but haven't pushed it
- I suspect I'm diffing the two the actual vs expected change.

Is that rough sketch correct? how do I integrate test for this feature with the existing test? I'm not seeing anything relevant to ScalarEvolution or analysis in llvm-test-suite when searching keywords

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