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

For functions where esi is used as base pointer, we would previously fall back
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