An upcoming loop vectorizer commit will want to replace a SCEVUnknown(Value*)
by a SCEVConstant. This commit modifies the SCEVParameterRewriter to support
this. The SCEVParameterRewriter constructor can optionally specify to follow
this behavior.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198949 91177308-0d34-0410-b5e6-96231b3b80d8