[turbofan] Avoid useless sign extension after sign extended load.

TEST=unittests
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/735623002

Cr-Commit-Position: refs/heads/master@{#25388}

src/compiler/machine-operator-reducer.cc

@@ -250,6 +250,21 @@ Reduction MachineOperatorReducer::Reduce(Node* node) {
       if (m.IsFoldable()) {                                  // K >> K => K
         return ReplaceInt32(m.left().Value() >> m.right().Value());
       }
+      if (m.left().IsWord32Shl()) {
+        Int32BinopMatcher mleft(m.left().node());
+        if (mleft.left().IsLoad()) {
+          LoadRepresentation const rep =
+              OpParameter<LoadRepresentation>(mleft.left().node());
+          if (m.right().Is(24) && mleft.right().Is(24) && rep == kMachInt8) {
+            // Load[kMachInt8] << 24 >> 24 => Load[kMachInt8]
+            return Replace(mleft.left().node());
+          }
+          if (m.right().Is(16) && mleft.right().Is(16) && rep == kMachInt16) {
+            // Load[kMachInt16] << 16 >> 16 => Load[kMachInt8]
+            return Replace(mleft.left().node());
+          }
+        }
+      }
       break;
     }
     case IrOpcode::kWord32Ror: {

test/unittests/compiler/machine-operator-reducer-unittest.cc

@@ -644,6 +644,36 @@ TEST_F(MachineOperatorReducerTest, Word32RorWithConstants) {
 }
 
 
+// -----------------------------------------------------------------------------
+// Word32Sar
+
+
+TEST_F(MachineOperatorReducerTest, Word32SarWithWord32ShlAndLoad) {
+  Node* const p0 = Parameter(0);
+  Node* const p1 = Parameter(1);
+  {
+    Node* const l = graph()->NewNode(machine()->Load(kMachInt8), p0, p1,
+                                     graph()->start(), graph()->start());
+    Reduction const r = Reduce(graph()->NewNode(
+        machine()->Word32Sar(),
+        graph()->NewNode(machine()->Word32Shl(), l, Int32Constant(24)),
+        Int32Constant(24)));
+    ASSERT_TRUE(r.Changed());
+    EXPECT_EQ(l, r.replacement());
+  }
+  {
+    Node* const l = graph()->NewNode(machine()->Load(kMachInt16), p0, p1,
+                                     graph()->start(), graph()->start());
+    Reduction const r = Reduce(graph()->NewNode(
+        machine()->Word32Sar(),
+        graph()->NewNode(machine()->Word32Shl(), l, Int32Constant(16)),
+        Int32Constant(16)));
+    ASSERT_TRUE(r.Changed());
+    EXPECT_EQ(l, r.replacement());
+  }
+}
+
+
 // -----------------------------------------------------------------------------
 // Word32Shl