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

src/compiler/arm64/code-generator-arm64.cc

Index: src/compiler/arm64/code-generator-arm64.cc
diff --git a/src/compiler/arm64/code-generator-arm64.cc b/src/compiler/arm64/code-generator-arm64.cc
index 2b8d40917b8f65790f4ea57ea0b4c5778ead4e54..d1d6a6d087eed7abe11bc14eb409be95935cdf5f 100644
--- a/src/compiler/arm64/code-generator-arm64.cc
+++ b/src/compiler/arm64/code-generator-arm64.cc
@@ -142,6 +142,9 @@ class Arm64OperandConverter FINAL : public InstructionOperandConverter {
         return Operand(constant.ToExternalReference());
       case Constant::kHeapObject:
         return Operand(constant.ToHeapObject());
+      case Constant::kRpoNumber:
+        UNREACHABLE();  // TODO(dcarney): RPO immediates on arm64.
+        break;
     }
     UNREACHABLE();
     return Operand(-1);