patch applied (ghc-6.8/ghc): MERGED: Fix Trac #1899; missing equality check in typechecker's constraint simplifier

[Ian Lynagh]

Sat Mar  1 06:53:06 PST 2008  Ian Lynagh <igloo at earth.li>
  * MERGED: Fix Trac #1899; missing equality check in typechecker's constraint simplifier
  simonpj at microsoft.com**20080226174743
   
   This patch fixes a missing equality check (uifying type variable b=b) in
   the new constraint simplifier in TcTyFuns.  As it stands, we were making
   'b' point to itself, which subsequently led to an infinite loop when
   zonking.  Test is T1899.hs

    M ./compiler/typecheck/TcTyFuns.lhs -14 +18