Given two unboxed sum terms:

    (# 1 | #) :: (# Int | Int# #)
    (# 1 | #) :: (# Int | Int  #)

These two terms are not equal as they unarise to different unboxed
tuples. However StgCse was thinking that these are equal, and replacing
one of these with a binder to the other.

To not deal with unboxed sums in StgCse we now do it after unarise. For
StgCse to maintain post-unarise invariants we factor-out case binder
in-scopeness check to `stgCaseBndrInScope` and use it in StgCse.

Also did some refactoring in SimplStg.

Another way to fix this would be adding a special case in StgCse to not
bring unboxed sum binders in scope:

    diff --git a/compiler/simplStg/StgCse.hs
b/compiler/simplStg/StgCse.hs
    index 6c740ca4cb..93a0f8f6ad 100644
    --- a/compiler/simplStg/StgCse.hs
    +++ b/compiler/simplStg/StgCse.hs
    @@ -332,7 +332,11 @@ stgCseExpr env (StgLetNoEscape binds body)
     stgCseAlt :: CseEnv -> OutId -> InStgAlt -> OutStgAlt
     stgCseAlt env case_bndr (DataAlt dataCon, args, rhs)
         = let (env1, args') = substBndrs env args
    -          env2 = addDataCon case_bndr dataCon (map StgVarArg
args') env1
    +          env2
    +            | isUnboxedSumCon dataCon
    +            = env1
    +            | otherwise
    +            = addDataCon case_bndr dataCon (map StgVarArg args')
env1
                 -- see note [Case 2: CSEing case binders]
               rhs' = stgCseExpr env2 rhs
           in (DataAlt dataCon, args', rhs')

I think this patch seems better in that it doesn't add a special case to
StgCse.

Test Plan:
Validate.

I tried to come up with a minimal example but failed. I thought a simple
program like

    data T = T (# Int | Int #) (# Int# | Int #)

    case T (# 1 | #) (# 1 | #) of ...

should be enough to trigger this bug, but for some reason StgCse
doesn't do
anything on this program.

Reviewers: simonpj, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15300

Differential Revision: https://phabricator.haskell.org/D4962

(cherry picked from commit 3c311e50)