GHC can't find weight for edge that should have one

Summary

GHC panics while compiling fetchAndIntArray# / fetchOrIntArray#.

[ 5 of 13] Compiling Data.Bit.MutableTS
ghc: panic! (the 'impossible' happened)
  (GHC version 8.8.1 for x86_64-apple-darwin):
  Can't find weight for edge that should have one
  triple (nQaY, nQwj, sKLs)
  updates [(cPjA, nQwR, sL5U), (cPjx, nQwQ, sL5U),
           ...
           (cOPS, nQwl, sKKV), (cOPy, nQwk, sKLs), (nQaY, nQwj, sKLs),
           ...
           (cOEj, nQv2, sKED), (cOEm, nQv1, sKED)]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1159:37 in ghc:Outputable
        pprPanic, called at compiler/nativeGen/CFG.hs:423:11 in ghc:CFG

Steps to reproduce

git clone https://github.com/Bodigrim/bitvec.git
git checkout bc84af009921802106239d5e963e00abd1ec7003
stack build 

This will fail with an error message, quoted above.

Sorry for such a broad description; at the moment I do not have time to whittle it down. I suspect that the problematic piece is in Data.Bit.InternalTS:

let W# andMask# = hiMask lenMod
    W# orMask#  = x .&. loMask lenMod
primitive $ \state ->
  let !(# state',  _ #) = fetchAndIntArray# mba loIx# (word2Int# andMask#) state  in
    let !(# state'', _ #) = fetchOrIntArray#  mba loIx# (word2Int# orMask#)  state' in
      (# state'', () #)

Expected behavior

I expect this package to compile successfully.

Environment

• GHC version used: 8.8.1 for x86_64-apple-darwin

It seems to be a regression since GHC 8.6.5, as witnessed by these Travis builds:

• GHC 8.6.5: https://travis-ci.org/Bodigrim/bitvec/jobs/595834070
• GHC 8.8.1: https://travis-ci.org/Bodigrim/bitvec/jobs/595834069