Merge register allocation fix for idiv from before-ghci-branch:

  1.59.2.1  +2 -37     fptools/ghc/compiler/nativeGen/MachCode.lhs
  1.40.2.1  +15 -4     fptools/ghc/compiler/nativeGen/MachMisc.lhs
  1.43.2.1  +20 -1     fptools/ghc/compiler/nativeGen/PprMach.lhs
  1.26.2.1  +8 -15     fptools/ghc/compiler/nativeGen/RegAllocInfo.lhs