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

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

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/code-generator.h"
 
 #include "src/arm64/macro-assembler-arm64.h"
 #include "src/compiler/code-generator-impl.h"
 #include "src/compiler/gap-resolver.h"
 #include "src/compiler/node-matchers.h"
@@ skipping 124 matching lines @@
       case Constant::kFloat32:
         return Operand(
             isolate()->factory()->NewNumber(constant.ToFloat32(), TENURED));
       case Constant::kFloat64:
         return Operand(
             isolate()->factory()->NewNumber(constant.ToFloat64(), TENURED));
       case Constant::kExternalReference:
         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);
   }
 
   MemOperand ToMemOperand(InstructionOperand* op, MacroAssembler* masm) const {
     DCHECK(op != NULL);
     DCHECK(!op->IsRegister());
     DCHECK(!op->IsDoubleRegister());
     DCHECK(op->IsStackSlot() || op->IsDoubleStackSlot());
@@ skipping 858 matching lines @@
     }
   }
   MarkLazyDeoptSite();
 }
 
 #undef __
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8