Make SCEVCallbackVH::allUsesReplacedWith update the old SCEVUnknown

object, as it may still be referenced by SCEVs not cleaned up by the
use list traversal.

Also, in ScalarEvolution::forgetValue, only check for a SCEVUnknown
object for the original value, not for any value in the use list,
because other SCEVUnknown values aren't necessary obsolete at that
point.

llvm-svn: 109570