[arm][arm64] Improve CountTrailingZero implementation.

src/arm/simulator-arm.cc

 // Copyright 2012 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 <stdarg.h>
 #include <stdlib.h>
 #include <cmath>
 
 #if V8_TARGET_ARCH_ARM
 
@@ skipping 2902 matching lines @@
                         rm_val = (rm_val >> 16) | (rm_val << 16);
                         break;
                       case 3:
                         rm_val = (rm_val >> 24) | (rm_val << 8);
                         break;
                     }
                     set_register(rd, rn_val + (rm_val & 0xFFFF));
                   }
                 }
               } else {
-                UNIMPLEMENTED();
+                // PU == 0b01, BW == 0b11, Bits(9, 6) != 0b0001
+                if ((instr->Bits(20, 16) == 0x1f) &&
+                    (instr->Bits(11, 4) == 0xf3)) {
+                  // Rbit.
+                  uint32_t rm_val = get_register(instr->RmValue());
+                  set_register(rd, base::bits::ReverseBits(rm_val));
+                } else {
+                  UNIMPLEMENTED();
+                }
               }
               break;
           }
         }
         return;
       }
       break;
     }
     case db_x: {
       if (instr->Bits(22, 20) == 0x5) {
@@ skipping 1281 matching lines @@
   set_register(sp, current_sp + sizeof(uintptr_t));
   return address;
 }
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // USE_SIMULATOR
 
 #endif  // V8_TARGET_ARCH_ARM