Auto merge of #139965 - amandasystems:marginally-improved-scc-annotations, r=lcnr

Decouple SCC annotations from SCCs

This rewires SCC annotations to have them be a separate, visitor-type data structure. It was broken out of #130227, which needed them to be able to remove unused annotations after computation without recomputing the SCCs themselves.

As a drive-by it also removes some redundant code from the hot loop in SCC construction for a performance improvement.

r? lcnr

Author: bors