Speed up conversion of untagged int32 result if we know it must be a smi.

src/ast.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 237 matching lines @@
 
   // True if the expression is a loop condition.
   bool is_loop_condition() const {
     return LoopConditionField::decode(bitfields_);
   }
   void set_is_loop_condition(bool flag) {
     bitfields_ = (bitfields_ & ~LoopConditionField::mask()) |
         LoopConditionField::encode(flag);
   }
 
+  // The value of the expression is guaranteed to be a smi, because the
+  // top operation is a bit operation with a mask, or a shift.
+  bool GuaranteedSmiResult();
+
   // AST analysis results
 
   // True if the expression rooted at this node can be compiled by the
   // side-effect free compiler.
   bool side_effect_free() { return SideEffectFreeField::decode(bitfields_); }
   void set_side_effect_free(bool is_side_effect_free) {
     bitfields_ &= ~SideEffectFreeField::mask();
     bitfields_ |= SideEffectFreeField::encode(is_side_effect_free);
   }
 
@@ skipping 1890 matching lines @@
 
   // Holds the result of copying an expression.
   Expression* expr_;
   // Holds the result of copying a statement.
   Statement* stmt_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_AST_H_