Add `edges` vector to CodeInstance/CodeInfo to keep backedges as edges (#54894)
Appears to add about 11MB (128MB to 139MB) to the system image, and to
decrease the stdlib size by 55 MB (325MB to 270MB), so seems overall
favorable right now. The edges are computed following the encoding
<https://hackmd.io/sjPig55kS4a5XNWC6HmKSg?both#Edges-Encoding> to
correctly reflect the backedges.
Co-authored-by: Shuhei Kadowaki <aviatesk@gmail.com>