[Bug target/49244] __sync or __atomic builtins will not emit 'lock bts/btr/btc'

[dhowells at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49244

--- Comment #7 from dhowells at redhat dot com <dhowells at redhat dot com> ---
We should also be able to reduce:

bool
test_bit (int *a, int bit)
{
  uint mask = (1u << bit);

  return (__atomic_load_n (a, __ATOMIC_xxx) & mask) != 0;
}

to a BT instruction on x86.