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

[William Moses via llvm-commits]

wsmoses wrote:

@nikic yeah I'm not sure willreturn will work in this case, since the original input function may not be willreturn. E.g. we don't re-optimize the function (including from other calling contexts) with willreturn, but this addition is only needed for cache size calculations which are only used if at runtime the function returned from the original code.

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