Revert "Revert "MachineScheduler: better book-keeping for asserts.""

This reverts commit r212109, which reverted r212088.

However, disable the assert as it's not necessary for correctness.  There are
several corner cases that the assert needed to handle better for in-order
scheduling, but none of them are incorrect scheduler behavior. The assert is
mainly there to collect good unit tests like this and ensure that the
target-independent scheduler is working as expected with the various machine
models.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212187 91177308-0d34-0410-b5e6-96231b3b80d8