Fix so that reaching definitions are properly computed in loop optimization when there is unreachable code.

Specifically, this ensures that the depth-first numbering of basic blocks starts from 1, which is what ReachingDefinitions expects. Without this fix, reaching definitions wouldn't be correctly computed for functions that contain unreachable basic blocks (including the implicit one to return at the end). This could result in invalid hoisting of operations out of the loop.

This fixes the compca26.c test case.

DAG.pas

@@ -3164,6 +3164,14 @@ var
       bb := bb^.next;
       end; {while}
    Search(DAGBlocks);
+   if i <> 0 then begin                 {ensure DFNs start from 1}
+      bb := DAGblocks;
+      while bb <> nil do begin
+         if bb ^.dfn <> 0 then
+            bb^.dfn := bb^.dfn - i;
+         bb := bb^.next;
+         end; {while}
+      end; {if}
    end; {DepthFirstOrder}