Fix in-scope set for CSE

Ticket #25468 showed an assertion failure in CSE because a top-level
Id was being used before it was defined.  Reason: Note [Glomming] in
GHC.Core.Opt.OccurAnal.

Solution (used in many places): just put all the top-level bindings in
scope at the beginning of CSE.

Compile-time allocation wobbles up and down a tiny bit; geo mean is
zero. But MultiLayerModulesTH_OneShot and hard_hole_fits increase (on
some architectures only) by a bit oever 2% .  I think these are just a
random fluctuations.

Metric Increase:
    MultiLayerModulesTH_OneShot
    hard_hole_fits