cfglceanup: Fix check for preheaders

cfglceanup: Fix check for preheaders

I had messed up the check in r16-5258-g1d8e2d51e5c5cb for preheaders
where return to remvoe the forwarder preheader block even if LOOPS_HAVE_PREHEADERS
was set. I am not sure how often this happens because most of the time the pre-header
will have an incoming phi block anyways but it is safer not to remove it in this case.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-cfgcleanup.cc (tree_forwarder_block_p): Restore check on
	LOOPS_HAVE_PREHEADERS.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>