[PATCH] memcpy lowering: use "rep movs" even when esi is used as base pointer

[Hans Wennborg]

  Ping?

  Since I'm not very confident in the sdag and mi area, I was hoping someone could verify that:

  1) I'm hooking up the sdag nodes correctly
  2) The MI layer will handle register conflicts (i.e. if something else is in %edx it will get spilled or allocated to a different register, etc.)

  Please take a look. Thanks.

http://llvm-reviews.chandlerc.com/D2968