[mlgo] Fix accounting for SCC splits

Previously if the inliner split an SCC such that an empty one remained, the MLInlineAdvisor could potentially lose track of the EdgeCount if a subsequent CGSCC pass modified the calls of a function that was initially in the SCC pre-split. Saving the seen nodes in onPassEntry resolves this.

Reviewed By: mtrofin

Differential Revision: https://reviews.llvm.org/D127693