to smaller `letrec' groups or even `let*'.
The goal of the change is to avoid performance surprises
when using internal definitions, especially when mixing
experessions with definitions.
It's a somewhat scary change. Besides the new transformation,
the optimizer pass's coordinate system for `letrec' (and sometimes
`let*') bindings changed.
No standard benchmarks were harmed during the making of this
optimization. (None were improved, either.)
