llvm-dg: make edge to return block have label 255
llvm-dg: make edge to return block have label 255

so that we differentiate between this artificial edge
and real CFG edges

Signed-off-by: Marek Chalupa <a05df156a69ad59faa9af9b666ef78009594c145@gmail.com>

mchalupa/dg,mchalupa/dg,mchalupa/dg,mchalupa/dg