Make SCEVCallbackVH::allUsesReplacedWith update the old SCEVUnknown

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109570 91177308-0d34-0410-b5e6-96231b3b80d8