pprPanic compiling `hashtables` package (GHC head)
## Summary
```
ghc: panic! (the 'impossible' happened)
(GHC version 8.11.0.20200620:
urk! lookup local fingerprint
EmptyElement
[cESfjY :-> (Key, c95c9bd2232682a3ff3db6095ed5ddb6),
iESflj :-> ($trModule, c3d17b474b3aa6c49cfa5f44523d660e),
iESfll :-> ($trModule3, 6c39022df23a345128f3011a58ca4831),
iESflm :-> ($trModule1, c9ad33973b0e938d4771a350975b57e1),
iESflo :-> ($trModule2, 25989dc4f3bb4b8db88b64b50717e508),
iESflp :-> ($trModule4, a5ea0043e4ed95827f3cc81e07d1e5cd),
iEShIk :-> ($tc'EmptyElement, 79562e12a6a10f125f99790de6fae5ae),
iEShIm :-> ($tcTombStone, b36edcacb6f137c85f02e04c068e68d6),
iEShIp :-> ($tc'EmptyElement1, 24291eaccc11012b9a2224cd79c979d7),
iEShIq :-> ($tc'EmptyElement2, 8ab16fee04b9ba027acabec4d15fec6a),
iEShIr :-> ($tc'EmptyElement3, c6f99d035bf964ce09d6be780e1a3c02),
iEShIs :-> ($tcTombStone1, 794a3fe1dfd8879bfa45ffeaac5fffec),
iEShIt :-> ($tcTombStone2, a70e6297f3446513cfc2f6d032b2f0e2)]
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler/GHC/Utils/Outputable.hs:1229:37 in ghc:GHC.Utils.Outputable
pprPanic, called at compiler/GHC/Iface/Recomp.hs:811:37 in ghc:GHC.Iface.Recomp
```
## Steps to reproduce
Minimal reproducer: [Repro.hs](/uploads/dff6bef210a11acfdf8cf8809811ad9c/Repro.hs). Compile with `-O2`.
## Expected behavior
No panic
## Environment
* GHC version used: Head commit d4a0be758003f32b9d9d89cfd14b9839ac002f4d
Optional:
* Operating System: FreeBSD 12
* System Architecture: x86_64
issue