This patch caused at least the following test failures:
   1744(normal)
   ghci028(ghci)
   unicode001(normal)
and additionally made the stage3 build fail.  

A little more validation please!

I didn't find the exact cause of the failure yet, but it appears that
the Lexer is miscompiled in some strange way.  If any of {Encoding,
StringBuffer, or Lexer} are compiled without -O, the problem goes
away.