[CodeGen] Allow mempcy/memset to generate small overlapping stores.

Summary:
All targets either just return false here or properly model `Fast`, so I
don't think there is any reason to prevent CodeGen from doing the right
thing here.

Subscribers: nemanjai, javed.absar, eraman, jsji, llvm-commits

Differential Revision: https://reviews.llvm.org/D55365

llvm-svn: 348843