Re-factor mkAtomicArgs and completeNonRecX

Re-factor mkAtomicArgs and completeNonRecX

This refactoring ensures that when mkAtomicArgs adds new bindings,
it does so using completeNonRecX, which adds unfoldings etc.  More
modular, and saves passes too.

(This was important when getting rules to work right.  We want tob
fire a rule as soon as possible, taking into account all inlinings,
else a less-good rule applies.  That's what I found when doing
stream fusion anyway.)

Regardless, this is an improvement.