[polly] Fix SCEVLoopAddRecRewriter to avoid invalid AddRecs.

When we're remapping an AddRec, the AddRec constructed by a partial
rewrite might not make sense.  This triggers an assertion complaining
it's not loop-invariant.

Instead of constructing the partially rewritten AddRec, just skip
straight to calling evaluateAtIteration.

Testcase was automatically reduced using llvm-reduce, so it's a little
messy, but hopefully makes sense.

Differential Revision: https://reviews.llvm.org/D102959