[arm][arm64] Improve CountTrailingZero implementation.

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

Index: src/compiler/arm/code-generator-arm.cc
diff --git a/src/compiler/arm/code-generator-arm.cc b/src/compiler/arm/code-generator-arm.cc
index 8e1dcf8e4902fe840b5729e7fdff857c8ff50d2d..9f3655740ea85b74fdaa40d342c7ab0e7bed2421 100644
--- a/src/compiler/arm/code-generator-arm.cc
+++ b/src/compiler/arm/code-generator-arm.cc
@@ -701,6 +701,12 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
                i.InputInt32(2));
       DCHECK_EQ(LeaveCC, i.OutputSBit());
       break;
+    case kArmRbit: {
+      CpuFeatureScope scope(masm(), ARMv7);
+      __ rbit(i.OutputRegister(), i.InputRegister(0));
+      DCHECK_EQ(LeaveCC, i.OutputSBit());
+      break;
+    }
     case kArmClz:
       __ clz(i.OutputRegister(), i.InputRegister(0));
       DCHECK_EQ(LeaveCC, i.OutputSBit());