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.