Remove start block special case

Edges to the start block are invalid. The special case is unnecessary.

Author: tmiasko

compiler/rustc_mir_transform/src/add_call_guards.rs

@@ -34,7 +34,6 @@ impl<'tcx> crate::MirPass<'tcx> for AddCallGuards {
 fnrun_pass(&self,_tcx:TyCtxt<'tcx>,body:&mutBody<'tcx>){
 letmut pred_count:IndexVec<_,_> =
  body.basic_blocks.predecessors().iter().map(|ps| ps.len()).collect();
- pred_count[START_BLOCK] += 1;
 
 // We need a place to store the new blocks generated
 letmut new_blocks = Vec::new();