[MemCpyOpt] Do move the memset, but look at its dest's dependencies.

[MemCpyOpt] Do move the memset, but look at its dest's dependencies.

In effect a partial revert of r237858, which was a dumb shortcut.
Looking at the dependencies of the destination should be the proper
fix: if the new memset would depend on anything other than itself,
the transformation isn't correct.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237874 91177308-0d34-0410-b5e6-96231b3b80d8