cfgcleanup: forwarder block, ignore bbs which merge with the predecessor

cfgcleanup: forwarder block, ignore bbs which merge with the predecessor

While moving mergephi's forwarder block removal over to cfgcleanup,
I noticed a few regressions due to removal of a forwarder block (correctly)
but the counts were not updated, instead let these blocks be handled by the merge_blocks
cleanup code.

gcc/ChangeLog:

	* tree-cfgcleanup.cc (tree_forwarder_block_p): Reject bb which has a single
	predecessor which has a single successor.

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