fix (semantics) of MutableStmtGraph.insertBefore(Stmt existingStmt, FallsThroughStmt newStmt)

[swissiety]

https://github.com/soot-oss/SootUp/pull/869/files#diff-c5721a80c02fabaa342480b1e4d00ff3269aa3bc5ca20c721c4d30c0ad3fac75R87
suggests that something is wrong with insertBefore(...).

semantic options:
a) All (Branching-) predecessors of existingStmt would flow to the newStmt and then it goes to the existingStmt.
b) inserts a newStmt in the FallsThrough flow but not in the predecessor flows branching to the oldStmt.

• it seems a) is more intuitive for insertBefore(...)
• and an easier option for b) than: removeEdge(...), putEdge(...) etc. should exist as well.