8319822: Use a linear-time algorithm for assert_different_registers()

src/hotspot/cpu/aarch64/register_aarch64.hpp

@@ -389,14 +389,14 @@ typedef AbstractRegSet<PRegister> PRegSet;
 
 template <>
 inline Register AbstractRegSet<Register>::first() {
-  uint32_t first = checked_cast<uint32_t>(_bitset & -_bitset);
-  return first != 0 ? as_Register(exact_log2(first)) : noreg;
+  if (_bitset == 0) { return noreg; }
+  return as_Register(count_trailing_zeros(_bitset));
 }
 
 template <>
 inline FloatRegister AbstractRegSet<FloatRegister>::first() {
-  uint32_t first = checked_cast<uint32_t>(_bitset & -_bitset);
-  return first != 0 ? as_FloatRegister(exact_log2(first)) : fnoreg;
+  if (_bitset == 0) { return fnoreg; }
+  return as_FloatRegister(count_trailing_zeros(_bitset));
 }
 
 inline Register as_Register(FloatRegister reg) {