Revert change 5989, which causes failures in some benchmarks.

src/hydrogen-instructions.h

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1362 matching lines @@
       case kMathRound:
       case kMathCeil:
         set_representation(Representation::Integer32());
         break;
       case kMathAbs:
         set_representation(Representation::Tagged());
         SetFlag(kFlexibleRepresentation);
         break;
       case kMathSqrt:
       case kMathPowHalf:
-      case kMathLog:
       default:
         set_representation(Representation::Double());
     }
     SetFlag(kUseGVN);
   }
 
   virtual void PrintDataTo(StringStream* stream) const;
 
   virtual HType CalculateInferredType() const;
 
   virtual HValue* EnsureAndPropagateNotMinusZero(BitVector* visited);
 
   virtual Representation RequiredInputRepresentation(int index) const {
     switch (op_) {
       case kMathFloor:
       case kMathRound:
       case kMathCeil:
       case kMathSqrt:
       case kMathPowHalf:
-      case kMathLog:
         return Representation::Double();
         break;
       case kMathAbs:
         return representation();
         break;
       default:
         return Representation::None();
     }
   }
 
@@ skipping 1484 matching lines @@
   HValue* object() const { return left(); }
   HValue* key() const { return right(); }
 };
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_