[SCEV] Make getPostIncExpr guaranteed to return AddRec

The current implementation of `getPostIncExpr` invokes `getAddExpr` for two recurrencies
and expects that it always returns it a recurrency. But this is not guaranteed to happen if we
have reached max recursion depth or refused to make SCEV simplification for other reasons.

This patch changes its implementation so that now it always returns SCEVAddRec without
relying on `getAddExpr`.

Differential Revision: https://reviews.llvm.org/D42953

llvm-svn: 324866