LoopVectorize: Make Value pointers that could be RAUW'ed a VH

The Value pointers we store in the induction variable list can be RAUW'ed by a
call to SCEVExpander::expandCodeFor, use a TrackingVH instead. Do the same thing
in some other places where we store pointers that could potentially be RAUW'ed.

Fixes PR16073.

llvm-svn: 182485