[mlgo][inline] Improve global state tracking

The global state refers to the number of the nodes currently in the
module, and the number of direct calls between nodes, across the
module.

Node counts are not a problem; edge counts are because we want strictly
the kind of edges that affect inlining (direct calls), and that is not
easily obtainable without iteration over the whole module.

This patch avoids relying on analysis invalidation because it turned out
to be too aggressive in some cases. It leverages the fact that Node
objects are stable - they do not get deleted while cgscc passes are
run over the module; and cgscc pass manager invariants.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D115847