* Add comments for peepholes

* Implement new peephole:
     // Peephole optimize the following instructions:
     // %t1 = cast {<...>} * %StructPtr to <ty> *
     //
     // Into: %t2 = getelementptr {<...>} * %StructPtr, <0, 0, 0, ...>
     //       %t1 = cast <eltype> * %t1 to <ty> *

  This peephole eliminated 9 evil casts in the health benchmark, and
  completely turned the addList method around.  :)

llvm-svn: 1085