Add zero-length check to memcpy/memset load store loop expansion

Add zero-length check to memcpy/memset load store loop expansion

Summary:
I was testing using this expansion logic in other cases besides
NVPTX, and found some runtime failures due to the lack of a check
for a zero length memcpy/memset before the loop. There is already
such a check in the memmove expansion code though.

Reviewers: hfinkel

Subscribers: jholewinski, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D34707

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306541 91177308-0d34-0410-b5e6-96231b3b80d8