Atomics: make use of the "cmpxchg weak" instruction.

Atomics: make use of the "cmpxchg weak" instruction.

This also simplifies the IR we create slightly: instead of working out
where success & failure should go manually, it turns out we can just
always jump to a success/failure block created for the purpose. Later
phases will sort out the mess without much difficulty.

llvm-svn: 210917