[turbofan] Refactor the compare-zero folding in instruction selector.

src/compiler/instruction-selector-impl.h

 // 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.
 
 #ifndef V8_COMPILER_INSTRUCTION_SELECTOR_IMPL_H_
 #define V8_COMPILER_INSTRUCTION_SELECTOR_IMPL_H_
 
 #include "src/compiler/instruction.h"
 #include "src/compiler/instruction-selector.h"
 #include "src/compiler/linkage.h"
@@ skipping 369 matching lines @@
   }
 
   void Commute() {
     DCHECK(!IsNone());
     condition_ = CommuteFlagsCondition(condition_);
   }
 
   void Overwrite(FlagsCondition condition) { condition_ = condition; }
 
   void OverwriteAndNegateIfEqual(FlagsCondition condition) {
+    DCHECK(condition_ == kEqual || condition_ == kNotEqual);
     bool negate = condition_ == kEqual;
     condition_ = condition;
     if (negate) Negate();
   }
 
   void OverwriteUnsignedIfSigned() {
     switch (condition_) {
       case kSignedLessThan:
         condition_ = kUnsignedLessThan;
         break;
@@ skipping 43 matching lines @@
                                  // or mode_ == kFlags_set.
   BasicBlock* true_block_;       // Only valid if mode_ == kFlags_branch.
   BasicBlock* false_block_;      // Only valid if mode_ == kFlags_branch.
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_INSTRUCTION_SELECTOR_IMPL_H_