[MachineScheduler] Minor refactoring.

Duplicated code found in three places put into a new static function:

/// Given a Count of resource usage and a Latency value, return true if a
/// SchedBoundary becomes resource limited.
static bool checkResourceLimit(unsigned LFactor, unsigned Count,
                               unsigned Latency) {
  return (int)(Count - (Latency * LFactor)) > (int)LFactor;
}

Review: Florian Hahn, Matthias Braun
https://reviews.llvm.org/D39235

llvm-svn: 316560