fannkuchredux slowdown after commit de9d3125

de9d3125 corrected the logic in x86 backend to decide whether an integer constant is a valid x86 immediate number (32 bits). It is intended to allow more constants as x86 immediate numbers. However, on mubench (http://squirrel.anu.edu.au/mubench/) it shows a slowdown for fannkuchredux after the commit. I need to investigate into this.