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

src/compiler/x64/code-generator-x64.cc

Index: src/compiler/x64/code-generator-x64.cc
diff --git a/src/compiler/x64/code-generator-x64.cc b/src/compiler/x64/code-generator-x64.cc
index 174160c02c757c10bf66f0449482a7a57ed2ab3b..f085c3e404ae862ffedc6e8ae774be44572b99e7 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -919,6 +919,9 @@ void CodeGenerator::AssembleMove(InstructionOperand* source,
         case Constant::kHeapObject:
           __ Move(dst, src.ToHeapObject());
           break;
+        case Constant::kRpoNumber:
+          UNREACHABLE();  // TODO(dcarney): load of labels on x64.
+          break;
       }
       if (destination->IsStackSlot()) {
         __ movq(g.ToOperand(destination), kScratchRegister);