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

src/compiler/machine-operator-reducer.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/machine-operator-reducer.h"
 
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
 #include "src/codegen.h"
 #include "src/compiler/diamond.h"
@@ skipping 232 matching lines @@
         return ReplaceInt32(m.left().Value() >> m.right().Value());
       }
       break;
     }
     case IrOpcode::kWord32Sar: {
       Int32BinopMatcher m(node);
       if (m.right().Is(0)) return Replace(m.left().node());  // x >> 0 => x
       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: {
       Int32BinopMatcher m(node);
       if (m.right().Is(0)) return Replace(m.left().node());  // x ror 0 => x
       if (m.IsFoldable()) {                                  // K ror K => K
         return ReplaceInt32(
             base::bits::RotateRight32(m.left().Value(), m.right().Value()));
       }
       break;
@@ skipping 535 matching lines @@
 MachineOperatorBuilder* MachineOperatorReducer::machine() const {
   return jsgraph()->machine();
 }
 
 
 Graph* MachineOperatorReducer::graph() const { return jsgraph()->graph(); }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8