[IndVars, RLEV] Support rewriting exit values in loops without known exits (prep work)

This is a prepatory patch for future work on support exit value rewriting in loops with a mixture of computable and non-computable exit counts.  The intention is to be "mostly NFC" - i.e. not enable any interesting new transforms - but in practice, there are some small output changes.

The test differences are caused by cases wherewhere getSCEVAtScope can simplify a single entry phi without needing any knowledge of the loop.

llvm-svn: 367485