[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

include/llvm/CodeGen/RegAllocPBQP.h

@@ -257,9 +257,6 @@ public:
     assert(RS >= this->RS && "A node's reduction state can not be downgraded");
     this->RS = RS;
   }
-  bool isSpillable() const {
-    return RS == NotProvablyAllocatable || RS == OptimallyReducible;
-  }
 
   void handleAddEdge(const MatrixMetadata& MD, bool Transpose) {
     DeniedOpts += Transpose ? MD.getWorstRow() : MD.getWorstCol();