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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237066 91177308-0d34-0410-b5e6-96231b3b80d8