Do not combine reduction operators in intragroup.

src/Futhark/Pass/ExtractKernels/Intragroup.hs

@@ -253,19 +253,22 @@ intraGroupStm stm@(Let pat aux e) = do
             =<< runDistNestT env (distributeMapBodyStms acc (bodyStms $ lambdaBody lam))
     Op (Screma w arrs form)
       | Just (scans, mapfun) <- isScanomapSOAC form,
+        -- FIXME: Futhark.CodeGen.ImpGen.GPU.Group.compileGroupOp
+        -- cannot handle multiple scan operators yet.
         Scan scanfun nes <- singleScan scans -> do
           let scanfun' = soacsLambdaToGPU scanfun
               mapfun' = soacsLambdaToGPU mapfun
           certifying (stmAuxCerts aux) $
             addStms =<< segScan lvl pat mempty w [SegBinOp Noncommutative scanfun' nes mempty] mapfun' arrs [] []
           parallelMin [w]
     Op (Screma w arrs form)
-      | Just (reds, map_lam) <- isRedomapSOAC form,
-        Reduce comm red_lam nes <- singleReduce reds -> do
-          let red_lam' = soacsLambdaToGPU red_lam
+      | Just (reds, map_lam) <- isRedomapSOAC form -> do
+          let onReduce (Reduce comm red_lam nes) =
+                SegBinOp comm (soacsLambdaToGPU red_lam) nes mempty
+              reds' = map onReduce reds
               map_lam' = soacsLambdaToGPU map_lam
           certifying (stmAuxCerts aux) $
-            addStms =<< segRed lvl pat mempty w [SegBinOp comm red_lam' nes mempty] map_lam' arrs [] []
+            addStms =<< segRed lvl pat mempty w reds' map_lam' arrs [] []
           parallelMin [w]
     Op (Screma w arrs form) ->
       -- This screma is too complicated for us to immediately do