[SelectionDAG] Update check in createOperands to reflect max() is a valid value.

The value returned by max() is the last valid value, adjust the
comparison accordingly.

The code added in D55073 creates TokenFactors with max() operands.

Reviewers: aemerson, efriedma, RKSimon, craig.topper

Reviewed By: aemerson

Differential Revision: https://reviews.llvm.org/D56738

llvm-svn: 351318