[MachineScheduler] improve reuse of 'releaseNode'method

The 'SchedBoundary::releaseNode' is merely invoked for releasing the Top/Bottom root nodes.
However,  'SchedBoundary::releasePending' uses its same logic to check if the Pending queue
has any releasable SUnit.
It is possible to slightly modify the body of the two, allowing re-use of the former ('releaseNode')
in the latter.

Patch by Lorenzo Casalino <lorenzo.casalino93@gmail.com>

Reviewers: MatzeB, fhahn, atrick

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D65506