MachineScheduler: add some book-keeping to fix an assert.

Fixe for Bug 20057 - Assertion failied in llvm::SUnit* llvm::SchedBoundary::pickOnlyChoice(): Assertion `i <= (HazardRec->getMaxLookAhead() + MaxObservedStall) && "permanent hazard"'

Thanks to Chad for the test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211865 91177308-0d34-0410-b5e6-96231b3b80d8