[DagCombine] Increase depth by number of operands to avoid a pathological compile time.

We're hitting a pathological compile-time case, profiled to be in
DagCombiner::visitTokenFactor and many inserts into a SmallPtrSet.
It looks like one of the paths around findBetterNeighborChains is not
capped and leads to this.

This patch resolves the issue. Looking for feedback if this solution
looks reasonable.

Differential Revision: https://reviews.llvm.org/D118877