Fix the check for an empty basic block to check for an empty SUnits

Fix the check for an empty basic block to check for an empty SUnits
array instead, since this is what the scheduler actually cares about.
And remove a check that is unnecessary, since it can assume that
SUnits isn't empty.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@62362 91177308-0d34-0410-b5e6-96231b3b80d8