[DAGCombiner] Avoid creating large tokenfactors in visitTokenFactor

When simplifying TokenFactors, we potentially iterate over all
operands of a large number of TokenFactors. This causes quadratic
compile times in some cases and the large token factors cause additional
scalability problems elsewhere.

This patch adds some limits to the number of nodes explored for the
cases mentioned above.

Reviewers: niravd, spatel, craig.topper

Reviewed By: niravd

Differential Revision: https://reviews.llvm.org/D61397

llvm-svn: 360171