Atomics: emit "cmpxchg weak" where possible

Most builtins date from before the "cmpxchg weak" was a gleam in the
C++ committee's eye, so fortunately not much needs to change. But a
few of them *do* acknowledge that failure is possible.

For these, we'll emit the usual cartesian product of cmpxchg
operations if we can't statically determine weakness.  CodeGen can
sort it out later if the function gets inlined.

The only other non-trivial aspect of this is (I think) that we emit
the scalar expression for "IsWeak" once, at the beginning, and
propagate its value through the successive blocks. There's not much in
it, but it's slightly more consistent with the existing handling of
FailureOrder.