Move more functions from compile.c to flowgraph.c

[iritkatriel]

Following #117494, there is now less coupling between compile.c and there are some functions that can move to, and belong better in, flowgraph.c. There include:

• stack effect functions
• _PyCompile_OptimizeCfg
• _PyCfg_FromInstructionSequence

Linked PRs

• gh-119744: move a few functions from compile.c to flowgraph.c #119745