GlobalISel: Don't fail translate on weak cmpxchg

The translation of cmpxchg added by
9481399c specifically skipped weak
cmpxchg due to not understanding the meaning. Weak cmpxchg was added
in 420a2168. As explained in the
commit message, the weak mode is implicit in how
ATOMIC_CMP_SWAP_WITH_SUCCESS is lowered. If it's expanded to a regular
ATOMIC_CMP_SWAP, it's replaced with a strong cmpxchg.

This handling seems weird to me, but this was already following the
DAG behavior. I would expect the strong IR instruction to not have the
boolean output. Failing that, I might expect the IRTranslator to emit
ATOMIC_CMP_SWAP and a constant for the boolean.