[arm][arm64] Improve CountTrailingZero implementation.

src/arm/disasm-arm.cc

 // Copyright 2011 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.
 
 // A Disassembler object is used to disassemble a block of code instruction by
 // instruction. The default implementation of the NameConverter object can be
 // overriden to modify register names or to do symbol lookup on addresses.
 //
 // The example below will disassemble a block of code and print it to stdout.
 //
@@ skipping 1170 matching lines @@
                       case 2:
                         Format(instr, "uxtah'cond 'rd, 'rn, 'rm, ror #16");
                         break;
                       case 3:
                         Format(instr, "uxtah'cond 'rd, 'rn, 'rm, ror #24");
                         break;
                     }
                   }
                 }
               } else {
-                UNREACHABLE();
+                // PU == 0b01, BW == 0b11, Bits(9, 6) != 0b0001
+                if ((instr->Bits(20, 16) == 0x1f) &&
+                    (instr->Bits(11, 4) == 0xf3)) {
+                  Format(instr, "rbit'cond 'rd, 'rm");
+                } else {
+                  UNREACHABLE();
+                }
               }
               break;
           }
         }
       }
       break;
     }
     case db_x: {
       if (instr->Bits(22, 20) == 0x5) {
         if (instr->Bits(7, 4) == 0x1) {
@@ skipping 776 matching lines @@
     v8::internal::PrintF(
         f, "%p    %08x      %s\n",
         prev_pc, *reinterpret_cast<int32_t*>(prev_pc), buffer.start());
   }
 }
 
 
 }  // namespace disasm
 
 #endif  // V8_TARGET_ARCH_ARM