MemCpyOpt: When merging memsets also merge the trivial case of two memsets with the same destination.

The testcase is from PR19092, but I think the bug described there is actually a clang issue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203489 91177308-0d34-0410-b5e6-96231b3b80d8