Merge produces `let ... in ...` inside `do` block

[sgraf812]

Here's a repro.zip that, when merged with hsreduce merge massiv-example produces an AllInOne.hs with a syntax error:

      go !i
        | i Prelude.< k4
        = do let
               r = f (unsafeLinearIndex_DataMassivCoreCommon arr i)
                     Prelude.||
                       f (unsafeLinearIndex_DataMassivCoreCommon arr (i Prelude.+ 1))
                         Prelude.||
                           f (unsafeLinearIndex_DataMassivCoreCommon arr (i Prelude.+ 2))
                             Prelude.||
                               f (unsafeLinearIndex_DataMassivCoreCommon arr (i Prelude.+ 3))
             in
               if r then
                   Control.Scheduler.cancelBatchWith batch Prelude.True
               else
                   do done <- Control.Scheduler.hasBatchFinished batch
                      if done then Prelude.pure Prelude.True else go (i Prelude.+ 4)

Compiling the file complains about the in. I think let ... in ... might be illegal in that situation because it produces an expression, but we need a statement in that context. Maybe this is a weakness in the Haskell grammar (after all, how to tell whether we want a statment let vs. expression let?), not sure.