New slow validate errors

[Ömer Sinan Ağacan]

Hi,

I see a lot of these errors in slow validate using current GHC HEAD:

    ghc: panic! (the 'impossible' happened)
      (GHC version 8.5.20180407 for x86_64-unknown-linux):
        Each block should be reachable from only one ProcPoint

This wasn't happening ~10 days ago. I suspect it may be D4417 but I haven't
checked.

Ömer