Use new sub-word coercion primops

src/RunGhc.hs

@@ -108,14 +108,19 @@ hsType W64 = "Word#"
 toHsWord :: Width -> String -> String
 toHsWord w x = "W# " <> parens (extendFn <> " " <> parens x)
   where
-#if defined(WORD_SIZE_32BIT)
+#if __GLASGOW_HASKELL__ >= 904
+    extendFn = "word" ++ show n ++ "ToWord#"
+    n = widthInBits w
+#elif defined(WORD_SIZE_32BIT)
     extendFn
-      | w == W32  = ""
+      | w == W32  = "word32ToWord#"
       | otherwise = "extendWord" <> show (widthBits w) <> "#"
-#else
+#elif defined(WORD_SIZE_64BIT)
     extendFn
       | w == W64  = ""
       | otherwise = "extendWord" <> show (widthBits w) <> "#"
+#else
+#error unknown word size
 #endif
 
 evalCmm :: EvalMethod -> Cmm -> IO Natural