[InstructionCost] Prevent InstructionCost being created with CostState.

For a function that returns InstructionCost, it is very tempting to write:

  return InstructionCost::Invalid;

But that actually returns InstructionCost(1 /* int value of Invalid */))
which has a totally different meaning. By marking this constructor as
`delete`, this can no longer happen.

llvm/include/llvm/Support/InstructionCost.h

@@ -47,6 +47,7 @@ private:
 public:
   InstructionCost() = default;
 
+  InstructionCost(CostState) = delete;
   InstructionCost(CostType Val) : Value(Val), State(Valid) {}
 
   static InstructionCost getInvalid(CostType Val = 0) {