CBE pass 2 invalidates proc points

The attached file is from reduced from NoFib's spectral/integer.

When I compile this with -O2 -fllvm, I get the following panic on HEAD:

ghc-stage2.exe: panic! (the 'impossible' happened)
  (GHC version 8.5.20180329 for x86_64-unknown-mingw32):
        Each block should be reachable from only one ProcPoint

FWIW, I don't have the LLVM toolchain installed, but this is still in GHC's backend.

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Windows
Architecture