[turbofan] Use special constant type for RPO block number operands.

src/compiler/instruction.cc

Index: src/compiler/instruction.cc
diff --git a/src/compiler/instruction.cc b/src/compiler/instruction.cc
index 3c617327f4614f972837aba787faa73820254081..aa0739e4068fd4c8670ac58e0c342849599908f6 100644
--- a/src/compiler/instruction.cc
+++ b/src/compiler/instruction.cc
@@ -332,6 +332,8 @@ std::ostream& operator<<(std::ostream& os, const Constant& constant) {
                        constant.ToExternalReference().address());
     case Constant::kHeapObject:
       return os << Brief(*constant.ToHeapObject());
+    case Constant::kRpoNumber:
+      return os << "RPO" << constant.ToRpoNumber().ToInt();
   }
   UNREACHABLE();
   return os;