Have rpoSortCfgWithIds compute RPO ids instead of PO ids.

Summary: I wanted to index into the RPO-sorted block vector using the ids but it
was needlessly verbose ("block[block.size() - poId - 1]"), and all of
our uses of PO ids boiled down to "id1 < id2", which easily translates
to "rpoId1 > rpoId2".

Turns out that the use of the ids in reflowTypes was wrong too, it was
using them to detect back-edges, but it assumed they were RPO ids.

Reviewed By: @swtaarrs

Differential Revision: D1672905

Signature: t1:1672905:1415730536:a98b2f5b8d64e9a1a3a046220f8f68807b0c02e0