MachineScheduler: Refactor setPolicy() to limit computing remaining latency

Summary:
Computing the remaining latency can be very expensive especially
on graphs of N nodes where the number of edges approaches N^2.

This reduces the compile time of a pathological case with the
AMDGPU backend from ~7.5 seconds to ~3 seconds.  This test case has
a basic block with 2655 stores, each with somewhere between 500
and 1500 successors and predecessors.

Reviewers: atrick, MatzeB, airlied, mareko

Reviewed By: mareko

Subscribers: tpr, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D50486

llvm-svn: 340346