[MemCpyOpt] Don't move the memset when optimizing memset+memcpy.

Fixes PR23599, another miscompile introduced by r235232: when there is
another dependency on the destination of the created memset (i.e., the
part of the original destination that the memcpy doesn't depend on)
between the memcpy and the original memset, we would insert the created
memset after the memcpy, and thus after the other dependency.

Instead, insert the created memset right after the old one.

llvm-svn: 237858