Make SCEVRewriteVisitor smarter about when it trys to create SCEVs

This change really saves just one foldingset lookup, but makes
SCEVRewriteVisitor "feature compatible" with the handwritten logic in
ScalarEvolutionNormalization, so that I can change
ScalarEvolutionNormalization to use SCEVRewriteVisitor in a next step.

This is a non-functional change, but _may_ improve performance in some
pathological cases, but that's unlikely.

llvm-svn: 300348