[Polly] Fix infinite recursion in `ScopExpander::visitAddRecExpr` (#206063)

visitUnknown follow VMap, call GenSE.getSCEV() on the mapped value, and
get back the original AddRec, recursing visitAddRecExpr infinitely.

Insert the result into SCEVCache before calling visit() to cut
recursion.

Fixes #205137.

GitOrigin-RevId: 6461bd3749dc09698b76c3349e7b4befab7f7f9b