[SCEV] Make PendingLoopPredicates more frugal; NFCI

I don't expect `PendingLoopPredicates` to have very many
elements (e.g. when -O3'ing the sqlite3 amalgamation,
`PendingLoopPredicates` has at most 3 elements).  So now we use a
`SmallPtrSet` for it instead of the more heavyweight `DenseSet`.

llvm-svn: 282511