[ConstraintElim] Use default depth for most calls of isNonNegative.

Helps to improve resuls in some cases, while being overall neutral with
respect to compile-time,
https://llvm-compile-time-tracker.com/compare.php?from=2c9b6c1b36b8185299de083c3058e0c1e7760442&to=5984b1649dc12741308089de235647cf036df95f&stat=instructions:u