improve cmpxchg

* remove @cmpxchg, add @cmpxchgWeak and @cmpxchgStrong
   - See explanations in the langref.
 * add operand type as first parameter
 * return type is ?T where T is the operand type

closes #461
f1f998e071Andrew Kelley committed on 4/18/2018, 4:16:42 PMparent96ebd8b
8 files changedLine totals unavailable