testsuite: Add test directive checking removal of link_error

This test needs a directive checking the removal of the link_error.
Committed as obvious.

Signed-off-by: Jennifer Schmitz <jschmitz@nvidia.com>

gcc/testsuite/
	* gcc.dg/tree-ssa/log_ident.c: Add scan for removal of
	link_error in optimized tree dump.

gcc/testsuite/gcc.dg/tree-ssa/log_ident.c

 /* { dg-do compile } */
-/* { dg-options "-O2 -ffast-math" } */
+/* { dg-options "-O2 -ffast-math -fdump-tree-optimized" } */
 /* { dg-require-effective-target c99_runtime } */
 
 extern void link_error(void);
@@ -54,3 +54,5 @@ extern void link_error(void);
 TEST_LOGS (double, , )
 TEST_LOGS (float, f, f)
 TEST_LOGS (long double, L, l)
+
+/* { dg-final { scan-tree-dump-not "link_error" "optimized" } } */