Add a comment explaining how DenseMap::insert works, because it is not

Add a comment explaining how DenseMap::insert works, because it is not
intuitive.
It does NOT update the value if the key is already in the map,
it also returns false if the key is already in the map, regardless
if the value matched.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83458 91177308-0d34-0410-b5e6-96231b3b80d8