Lett users of sparse propagation do their own thing with phi nodes if they want

Lett users of sparse propagation do their own thing with phi nodes if they want
to. This can be combined with LCSSA or SSI form to store more information on a
PHINode than can be computed by looking at its incoming values.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82317 91177308-0d34-0410-b5e6-96231b3b80d8