Do not short-circuit into free blocks in SegOps.

Not completely sure this is the right way to do it, but it works.

src/Futhark/Optimise/ArrayShortCircuiting/ArrayCoalescing.hs

@@ -421,8 +421,10 @@ shortCircuitSegOpHelper num_reds lvlOK lvl lutab pat@(Pat ps0) pat_certs space0
                 (aggSummaryMapPartial (scalarTable td_env) $ unSegSpace space0)
                 (S.toList s)
             Undeterminable -> pure Undeterminable
-        let res = noMemOverlap td_env destination_uses source_writes
-        if res
+        -- Do not allow short-circuiting from a segop-local memory
+        -- block (not in the topdown scope) to an outer memory block.
+        if dstmem entry `M.member` scope td_env
+          && noMemOverlap td_env destination_uses source_writes
           then pure bu_env_f
           else do
             let (ac, inh) = markFailedCoal (activeCoals bu_env_f, inhibit bu_env_f) k

tests/issue1774.fut

@@ -1,9 +1,10 @@
 -- ==
--- structure mc-mem { Alloc 2 }
+-- compiled random input { [3]f32 [100]f32 [10]f32 } auto output
+-- structure mc-mem { Alloc 3 }
 
 let f [n] [m] [l] (xs: [n]f32) (ys: [m]f32) (zs: *[l]f32) : [n][m]f32 =
   map (\x ->
-	 map (\y -> f32.sum (map (*y) (map (*x) zs)))
+         map (\y -> f32.sum (map (*y) (map (*x) zs)))
              ys)
       xs