gh-106149: move CFG and basicblock definitions into flowgraph.c, use them as opaque types in compile.c

[iritkatriel]

This moves all the code that manipulates the CFG to flowgraph.c.

• Issue: tidy up the division of work between compile.c and flowgraph.c #106149

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @iritkatriel for commit c05fa8e 🤖

If you want to schedule another build, you need to add the 🔨 test-with-refleak-buildbots label again.

[iritkatriel]

The ref leaks failures are unrelated to this PR.

[markshannon]

Looks good to me, although I've not checked every detail as I assume it is a refactor, with no change to behavior.

@iritkatriel Can you confirm that this is strictly a refactor?

[iritkatriel]

@iritkatriel Can you confirm that this is strictly a refactor?

Yes

It basically moves code from compile.c to flowgraph.c, and struct definitions from the header into flowgraph.c, and then changes some of the functions from private (declared in the header) to static in flowgraph.c.