CSE: Walk past join point lambdas (#15002)

As the CSE transformation traverses the syntax tree, it needs to go past
the lambdas of a join point, and only look for CSE opportunities inside,
as a join point’s lambdas must be preserved. Simple fix; comes with a
Note and a test case.

Thanks to Ryan Scott for an excellently minimized test case, and for
bisecting GHC.

Differential Revision: https://phabricator.haskell.org/D4572

(cherry picked from commit ae0cff0a)