The original patch said:

* I made some changes to the way in which eta expansion happens in
  CorePrep, mainly to ensure that *arguments* that become let-bound
  are also eta-expanded.  I'm still not too happy with the clarity
  and robustness fo the result.
  
Unfortunately this change apparently broke some invariants that were
relied on elsewhere, and in particular lead to panics when compiling
with profiling on.

Will re-investigate in the new year.