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

[William Moses via llvm-commits]

wsmoses wrote:

I'm also a fan of the MustExitScalarEvolution functionality being merged upstream as the proper solution (e.g. perhaps a flag that indicates that SE can assume all loops terminate, independent of the existence of LLVM Loop metadata to specify).

For context, the usage of MustExitScalarEvolution within Enzyme, is that unlike regular SE which has additional checks for exiting, users of MustExitSE are performing analysis within a context where these checks are unnecessary (and substantially decremental for performance). Specifically they are used to compute the maximum number of values from the original program which may need to be explicitly preserved. A separate check for termination is not needed, since the newly generated code will only run after the original code (and thus an infinite loop will never result in the generated code being run). Failure to deduce a fixed loop size, however, results in the values being stored in a dynamically resized list rather than a fixed-size array.

@skewballfox since 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 revoking my approval and this should not be merged until the discussion has concluded.

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