This patch extends SCEVParameterRewriter to support rewriting unknown
epxressions to arbitrary SCEV expressions. It will be used by further
I don't really have the context to evaluate this fully, so I'm mostly deferring to author. I don't see any obvious problems, and the generalization does look useful. I just don't know the callsite well enough to be certain this has the desired effect, and thus the need to defer to author.