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

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

Index: src/compiler/mips/code-generator-mips.cc
diff --git a/src/compiler/mips/code-generator-mips.cc b/src/compiler/mips/code-generator-mips.cc
index 4eb372a7800be2be7a6448bfeba6c2aa71acf226..6fb0a30edd41a2261f73140a882eda1e376af8e2 100644
--- a/src/compiler/mips/code-generator-mips.cc
+++ b/src/compiler/mips/code-generator-mips.cc
@@ -70,6 +70,9 @@ class MipsOperandConverter FINAL : public InstructionOperandConverter {
         // TODO(plind): Maybe we should handle ExtRef & HeapObj here?
         //    maybe not done on arm due to const pool ??
         break;
+      case Constant::kRpoNumber:
+        UNREACHABLE();  // TODO(titzer): RPO immediates on mips?
+        break;
     }
     UNREACHABLE();
     return Operand(zero_reg);
@@ -796,6 +799,9 @@ void CodeGenerator::AssembleMove(InstructionOperand* source,
         case Constant::kHeapObject:
           __ li(dst, src.ToHeapObject());
           break;
+        case Constant::kRpoNumber:
+          UNREACHABLE();  // TODO(titzer): loading RPO numbers on mips.
+          break;
       }
       if (destination->IsStackSlot()) __ sw(dst, g.ToMemOperand(destination));
     } else if (src.type() == Constant::kFloat32) {