tree-optimization/104716 - check if we can copy loop in loop distribution

The following checks whether we can copy the loop before attempting
to do so in loop distribution.  In the testcase there's a computed
goto and thus abnormal edges which we cannot redirect.

2022-03-01  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104716
	* tree-loop-distribution.cc (find_seed_stmts_for_distribution):
	Check if we can copy the loop.

	* gfortran.dg/pr104716.f: New testcase.

gcc/testsuite/gfortran.dg/pr104716.f

@@ -0,0 +1,31 @@
+! { dg-do compile }
+! { dg-options "-std=legacy -O2 -ftree-loop-distribution -fno-move-loop-stores -fno-tree-dominator-opts" }
+
+      SUBROUTINE FOO()
+      
+      COMMON /WORK/ C2(2, 2)
+      
+      DIMENSION D11(2)
+
+      EQUIVALENCE (D11(1), C2(1, 1))
+
+      DO 40 I = 1, 2
+         DO 30 J = 1, 2
+            ASSIGN 10 TO ILBL
+            IF (C2(J, I) .NE. 0.0) THEN
+               ASSIGN 20 TO ILBL
+            ENDIF
+            GO TO ILBL
+ 10         CONTINUE
+ 20         CONTINUE
+            C2(J, I) = C2(J, I) + 1
+ 30      CONTINUE
+ 40   CONTINUE
+
+      DO 50 I = 1, 2
+         PRINT 90, I
+ 50   CONTINUE
+      
+      RETURN
+ 90   FORMAT(I5)
+      END

gcc/tree-loop-distribution.cc

@@ -3277,8 +3277,15 @@ find_seed_stmts_for_distribution (class loop *loop, vec<gimple *> *work_list)
 	  work_list->safe_push (stmt);
 	}
     }
+  bool res = work_list->length () > 0;
+  if (res && !can_copy_bbs_p (bbs, loop->num_nodes))
+    {
+      if (dump_file && (dump_flags & TDF_DETAILS))
+	fprintf (dump_file, "cannot copy loop %d.\n", loop->num);
+      res = false;
+    }
   free (bbs);
-  return work_list->length () > 0;
+  return res;
 }
 
 /* A helper function for generate_{rawmemchr,strlen}_builtin functions in order