Add regression test for T19474.

In version 0.12.2.0 of vector when used with GHC-9.0 we
rebox values from storeable mutable vectors.

This should catch such a change in the future.

testsuite/tests/perf/should_run/T19474.hs

+module T19474 where
+
+import Control.Monad
+import qualified Data.Vector.Storable.Mutable as M
+
+main = do
+  v <- M.new 1
+  forM_ [0..10000] $ \some -> do
+    addSome v 0 some
+  M.unsafeRead v 0 >>= print
+
+addSome :: M.IOVector Int -> Int -> Int -> IO ()
+addSome m idx some = do
+    val <- M.unsafeRead m idx
+    M.unsafeWrite m  idx $ val + some

testsuite/tests/perf/should_run/all.T

@@ -395,3 +395,7 @@ test('T19347',
     compile_and_run,
     ['-O'])
 
+test ('T19474',
+      [ collect_stats('bytes allocated', 5), reqlib('vector') ],
+      compile_and_run,
+      ['-v0 -O'])