Clarify that duplicate edges are not allowed in the Machine CFG.

LLVM IR has labeled duplicate CFG edges, but since Machine CFG edges
don't have labels, it doesn't make sense to allow duplicates. There is
no way of telling what the edges mean.

Duplicate CFG edges cause confusion when dealing with edge weights. It
seems that code producing duplicate CFG edges usually does the wrong
thing with edge weights.

llvm-svn: 162227