GHC.Base: Use thenIO in instance Applicative IO

Since recent changes to CSE, the previous definition were no longer CSEd
with thenIO, which resulted in extra steps in the simplifier and hence
slightly larger compile times. See ticket:11781#comment:7.

Differential Revision: https://phabricator.haskell.org/D2092

libraries/base/GHC/Base.hs

@@ -1086,9 +1086,9 @@ instance  Functor IO where
 instance Applicative IO where
     {-# INLINE pure #-}
     {-# INLINE (*>) #-}
-    pure   = returnIO
-    m *> k = m >>= \ _ -> k
-    (<*>)  = ap
+    pure  = returnIO
+    (*>)  = thenIO
+    (<*>) = ap
 
 instance  Monad IO  where
     {-# INLINE (>>)   #-}

testsuite/tests/perf/compiler/all.T

@@ -624,7 +624,7 @@ test('T9020',
           [(wordsize(32), 343005716, 10),
            # Original:    381360728
            # 2014-07-31:  343005716 (Windows) (general round of updates)
-           (wordsize(64), 852298336, 10)])
+           (wordsize(64), 698401736, 10)])
            # prev:        795469104
            # 2014-07-17:  728263536 (general round of updates)
            # 2014-09-10:  785871680 post-AMP-cleanup
@@ -632,6 +632,7 @@ test('T9020',
            # 2015-10-21:  786189008 Make stronglyConnCompFromEdgedVertices deterministic
            # 2016-01-26:  698401736 improvement from using ExpTypes instead of ReturnTvs
            # 2016-04-06:  852298336 Refactoring of CSE #11781
+           # 2016-04-06:  698401736 Use thenIO in Applicative IO
       ],
       compile,[''])