GitLab

 * The old Reg type is now split into VirtualReg and RealReg.
 * For the graph coloring allocator, the type of the register graph
   is now (Graph VirtualReg RegClass RealReg), which shows that it colors
   in nodes representing virtual regs with colors representing real regs.
   (as was intended)  
 * RealReg contains two contructors, RealRegSingle and RealRegPair,
   where RealRegPair is used to represent a SPARC double reg 
   constructed from two single precision FP regs. 
 * On SPARC we can now allocate double regs into an arbitrary register
   pair, instead of reserving some reg ranges to only hold float/double values. 

  - nativeGen/Instruction defines a type class for a generic
    instruction set. Each of the instruction sets we have, 
    X86, PPC and SPARC are instances of it.
  
  - The register alloctors use this type class when they need
    info about a certain register or instruction, such as
    regUsage, mkSpillInstr, mkJumpInstr, patchRegs..
  
  - nativeGen/Platform defines some data types enumerating
    the architectures and operating systems supported by the 
    native code generator.
  
  - DynFlags now keeps track of the current build platform, and 
    the PositionIndependentCode module uses this to decide what
    to do instead of relying of #ifdefs.
  
  - It's not totally retargetable yet. Some info info about the
    build target is still hardwired, but I've tried to contain
    most of it to a single module, TargetRegs.
  
  - Moved the SPILL and RELOAD instructions into LiveInstr.
  
  - Reg and RegClass now have their own modules, and are shared
    across all architectures.