Fix validate with old bootstrapping compilers

They weren't smart enough to see our GADT pattern matches are complete,
so gave a warning.

compiler/cmm/CmmUtils.hs

 {-# LANGUAGE GADTs #-}
 {-# OPTIONS_GHC -fno-warn-deprecations #-}
 -- Warnings from deprecated blockToNodeList
+{-# OPTIONS_GHC -fno-warn-incomplete-patterns #-}
+#if __GLASGOW_HASKELL__ >= 703
+-- GHC 7.0.1 improved incomplete pattern warnings with GADTs
+{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
+#endif
 
 
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