[PBQP] Conservativelly allocatable nodes can be spilled and give a better solution

[PBQP] Conservativelly allocatable nodes can be spilled and give a better solution

Although such nodes are allocatable, the cost of spilling may be less than
allocating to register, so spilling the node may provide a better solution.
The assert does not account for this case, so remove it for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229103 91177308-0d34-0410-b5e6-96231b3b80d8