Spanning tree instrumentation (#47959)

Spanning tree instrumentation (#47959)

Add a new instrumentation mode that only instruments a subset of the edges in
the control flow graph. This reduces the total number of counters and so has
less compile time and runtime overhead than instrumenting every block.

Add a matching count reconstruction algorithm that recovers the missing edge
counts and all block counts.

See #46882 for more details on this approach.

Also in runtime pgo support, add the header offset to the copy source.
This fixes #47930.