[ExpandReductions] Don't push all intrinsics to the worklist. Just push reductions.

We were previously pushing all intrinsics used in a function to the
worklist. This is wasteful for memory in a function with a lot of
intrinsics.

We also ask TTI if we should expand every intrinsic, but we only
have expansion support for the reduction intrinsics. This just
wastes time for the non-reduction intrinsics.

This patch only pushes reduction intrinsics into the worklist and
skips other intrinsics.

Differential Revision: https://reviews.llvm.org/D69470