[GHC] #14989: "Each block should be reachable from only one ProcPoint" compiling `integer` with `-fllvm`

[GHC]

#14989: "Each block should be reachable from only one ProcPoint" compiling
`integer` with `-fllvm`
-------------------------------------+-------------------------------------
           Reporter:  sgraf          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Windows
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 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.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14989>
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