[MemCpyOpt] Look at any dependency -not just source- for memset+memcpy.

This fixes another miscompile introduced by r235232: when there was a
dependency on the memcpy destination other than the memset, we would
ignore it, because we only looked at the source dependency.

It was a mistake to use SrcDepInfo.  Instead, just use DepInfo.

llvm-svn: 237066