[SCEV] Limit AddRec "simplifications" to avoid combinatorial explosions

SCEV's transform that turns `{A1,+,A2,+,...,+,An}<L> * {B1,+,B2,+,...,+,Bn}<L>` into

a single AddRec of size `2n+1` with complex combinatorial coefficients can easily

trigger exponential growth of the SCEV (in case if nothing gets folded and simplified).

We tried to restrain this transform using the option `scalar-evolution-max-add-rec-size`,

but its default value seems to be insufficiently small: the test attached to this patch

with default value of this option `16` has a SCEV of >3M symbols (when printed out).

This patch reduces the simplification limit. It is not a cure to combinatorial

explosions, but at least it reduces this corner case to something more or less

reasonable.

Differential Revision: https://reviews.llvm.org/D53282

Reviewed By: sanjoy