llvm-slicer: ensure that sliced function has entry without predecessors
llvm-slicer: ensure that sliced function has entry without predecessors

LLVM don't allow to have predecessors for entry block, so just
create new one if we slice away the original one

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

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