[SCEV] Introduce SCEVUse wrapper type (NFC)
Add SCEVUse as a PointerIntPair wrapper around const SCEV * to prepare
for storing additional per-use information.
This commit contains the mechanical changes of adding an intial SCEVUse
wrapper and updating all relevant interfaces to take SCEVUse. Note that
currently the integer part is never set, and all SCEVUses are
considered canonical.
