MIPS64: Fix corner case for Word64And(Word64Shr(val,0)) reduction

src/compiler/mips64/instruction-selector-mips64.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/base/adapters.h"
 #include "src/base/bits.h"
 #include "src/compiler/instruction-selector-impl.h"
 #include "src/compiler/node-matchers.h"
 #include "src/compiler/node-properties.h"
 
@@ skipping 544 matching lines @@
       if (mleft.right().HasValue()) {
         // Any shift value can match; int64 shifts use `value % 64`.
         uint32_t lsb = static_cast<uint32_t>(mleft.right().Value() & 0x3f);
 
         // Dext cannot extract bits past the register size, however since
         // shifting the original value would have introduced some zeros we can
         // still use Dext with a smaller mask and the remaining bits will be
         // zeros.
         if (lsb + mask_width > 64) mask_width = 64 - lsb;
 
-        Emit(kMips64Dext, g.DefineAsRegister(node),
-             g.UseRegister(mleft.left().node()), g.TempImmediate(lsb),
-             g.TempImmediate(static_cast<int32_t>(mask_width)));
+        if (lsb == 0 && mask_width == 64) {
+          Emit(kArchNop, g.DefineSameAsFirst(node), g.Use(mleft.left().node()));
+        } else {
+          Emit(kMips64Dext, g.DefineAsRegister(node),
+               g.UseRegister(mleft.left().node()), g.TempImmediate(lsb),
+               g.TempImmediate(static_cast<int32_t>(mask_width)));
+        }
         return;
       }
       // Other cases fall through to the normal And operation.
     }
   }
   if (m.right().HasValue()) {
     uint64_t mask = m.right().Value();
     uint32_t shift = base::bits::CountPopulation64(~mask);
     uint32_t msb = base::bits::CountLeadingZeros64(~mask);
     if (shift != 0 && shift < 32 && msb + shift == 64) {
@@ skipping 2082 matching lines @@
   } else {
     DCHECK(kArchVariant == kMips64r2);
     return MachineOperatorBuilder::AlignmentRequirements::
         NoUnalignedAccessSupport();
   }
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8