Another folding problem: if a node r/w chain or flag, don't fold it if it

has already been selected. The number of use check is not strong enough since
a node can be replaced with newly created target node. e.g. If the original
node has two uses, when it is selected for one of the uses it is replaced with
another. Each node now has a single use but isel still should not fold it.

llvm-svn: 25651