[arm][arm64] Improve CountTrailingZero implementation.

src/arm/assembler-arm.cc

Index: src/arm/assembler-arm.cc
diff --git a/src/arm/assembler-arm.cc b/src/arm/assembler-arm.cc
index be85b96419798dade68820d4cae4ed93be1d7d48..cfdaea0b1d4a9bee9dd128cdad0a3a6243674f08 100644
--- a/src/arm/assembler-arm.cc
+++ b/src/arm/assembler-arm.cc
@@ -1947,6 +1947,16 @@ void Assembler::uxtah(Register dst, Register src1, Register src2, int rotate,
 }
 
 
+void Assembler::rbit(Register dst, Register src, Condition cond) {
+  // Instruction details available in ARM DDI 0406C.b, A8.8.144.
+  // cond(31-28) | 011011111111(27-16) | Rd(15-12) | 11110011(11-4) | Rm(3-0)
+  DCHECK(IsEnabled(ARMv7));
+  DCHECK(!dst.is(pc));
+  DCHECK(!src.is(pc));
+  emit(cond | 0x6FF * B16 | dst.code() * B12 | 0xF3 * B4 | src.code());
+}
+
+
 // Status register access instructions.
 void Assembler::mrs(Register dst, SRegister s, Condition cond) {
   DCHECK(!dst.is(pc));