Remove start block special case

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

compiler/rustc_mir_transform/src/add_call_guards.rs

@@ -34,7 +34,6 @@ impl<'tcx> crate::MirPass<'tcx> for AddCallGuards {
     fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         let mut 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
         let mut new_blocks = Vec::new();