Improve performance of compilation by ~8% by speeding up the

hlo rematerialization pass.

Changes:
. Wrap each HloInstruction* inside an Item structure that keeps
  associated data.  This allows us to get rid of a bunch of
  hash tables indexed by HloInstruction*.
* Switch to an intrusive linked list (instead of std::list) so
  that we can avoid a hash table that maps to std::list::iterator.
* Use inlined vector in a few places.
PiperOrigin-RevId: 163848365