Revert "X86 memcpy lowering: use "rep movs" even when esi is used as base pointer" (r204174)

>  For functions where esi is used as base pointer, we would previously fall ba
>  from lowering memcpy with "rep movs" because that clobbers esi.
>
>  With this patch, we just store esi in another physical register, and restore
>  it afterwards. This adds a little bit of register preassure, but the more
>  efficient memcpy should be worth it.
>
>  Differential Revision: http://llvm-reviews.chandlerc.com/D2968

This didn't work. I was ending up with code like this:

  lea     edi,[esi+38h]
  mov     ecx,0Fh
  mov     edx,esi
  mov     esi,ebx
  rep movs dword ptr es:[edi],dword ptr [esi]
  lea     ecx,[esi+74h] <-- Ooops, we're now using esi before restoring it from edx.
  add     ebx,3Ch
  mov     esi,edx

I guess if we want to do this we need stronger glue or something, or doing the expansion
much later.