In ScalarEvolution::forgetValue, eliminate any SCEVUnknown

In ScalarEvolution::forgetValue, eliminate any SCEVUnknown
entries associated with the value being erased in the
folding set map.  These entries used to be harmless, because
a SCEVUnknown doesn't store any information about its Value*,
so having a new Value allocated at the old Value's address
wasn't a problem. But now that ScalarEvolution is storing more
information about values, this is no longer safe.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107316 91177308-0d34-0410-b5e6-96231b3b80d8