Do not collapse goto chains beginning with the start block
Do not collapse goto chains beginning with the start block

If any block on a goto chain has more than one predecessor, then the new
start block would have basic block predecessors.

Skip the transformation for the start block altogether, to avoid
violating the new invariant that the start block does not have any basic
block predecessors.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust