A64: tidy up the disassembler

src/a64/disasm-a64.cc

Index: src/a64/disasm-a64.cc
diff --git a/src/a64/disasm-a64.cc b/src/a64/disasm-a64.cc
index 5ef75d55e2061c5d1ab40525a489cfb237913e5f..56248c904bb5c01d2876c92428f81c62880068bb 100644
--- a/src/a64/disasm-a64.cc
+++ b/src/a64/disasm-a64.cc
@@ -1487,7 +1487,7 @@ int Disassembler::SubstituteImmediateField(Instruction* instr,
       return 6;
     }
     default: {
-      UNIMPLEMENTED();
+      UNREACHABLE();
       return 0;
     }
   }
@@ -1563,7 +1563,7 @@ int Disassembler::SubstituteShiftField(Instruction* instr, const char* format) {
       return 3;
     }
     default:
-      UNIMPLEMENTED();
+      UNREACHABLE();
       return 0;
   }
 }
@@ -1625,7 +1625,7 @@ int Disassembler::SubstituteBranchTargetField(Instruction* instr,
     case 'm': offset = instr->ImmCmpBranch(); break;
     // BImmTest - test and branch immediate.
     case 'e': offset = instr->ImmTestBranch(); break;
-    default: UNIMPLEMENTED();
+    default: UNREACHABLE();
   }
   offset <<= kInstructionSizeLog2;
   char sign = '+';