patch applied (ghc): Tell the NCG that XOR foo, foo does *not* read foo

[Wolfgang Thaller]

Thu Dec  7 05:17:21 PST 2006  wolfgang.thaller at gmx.net
  * Tell the NCG that XOR foo, foo does *not* read foo
  
  On x86[-64], MachCodeGen uses the old XOR trick to zero a register.
  This patch makes regUsage not list the register as an input in this
  case.
  Listing the register as an input for the instruction could make it 
  appear like an unitialised value, which is bad because unitialised
  values can cause the register allocator to panic (at least in the
  presence of a loop).
  

    M ./compiler/nativeGen/RegAllocInfo.hs +2