[ScalarEvolution] Factor out RewriteMap utilities in applyLoopGuards (NFC)

This factors out two utilities used with RewriteMap in applyLoopGuards:
 - AddRewrite, which puts a rewrite rule in the map and if needed registers
   the rewrite in the list of rewritten expressions,
 - GetMaybeRewritten, which checks whether an expression has already been
   rewritten, and if so, returns the rewrite. Otherwise, returns the given
   expression.

This may be needed when adding new rewrite rules as not to copy-paste this
code.