[DAGISel] When checking if chain node is foldable, make sure the intermediate nodes...

[DAGISel] When checking if chain node is foldable, make sure the intermediate nodes have a single use across all results not just the result that was used to reach the chain node.

This recovers a test case that was severely broken by r296476, my making sure we don't create ADD/ADC that loads and stores when there is also a flag dependency.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296486 91177308-0d34-0410-b5e6-96231b3b80d8