[Intrinsics] Added writeonly attribute to the first arg of llvm.memmove

D18714 introduced writeonly attribute:

"Also start using the attribute for memset, memcpy, and memmove intrinsics,
and remove their special-casing in BasicAliasAnalysis."

But actually, writeonly was not attached to memmove - oversight, it seems.

So let's add it. As we can see, this helps DSE to eliminate redundant stores.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D89724