Clean up loop-closed PHIs after loop finalize

Clean up loop-closed PHIs after loop finalize

This patch propagates loop-closed PHIs them out at
loop_optimizer_finalize.  For some cases, to clean up loop-closed PHIs
would save efforts of optimization passes after loopdone.

Thanks,
Jiufu Guo.

gcc/ChangeLog:
2020-10-18  Jiufu Guo   <guojiufu@linux.ibm.com>

	* cfgloop.h (loop_optimizer_finalize): Add flag argument.
	* loop-init.c (loop_optimizer_finalize): Call clean_up_loop_closed_phi.
	* tree-cfgcleanup.h (clean_up_loop_closed_phi): New declare.
	* tree-ssa-loop.c (tree_ssa_loop_done): Call loop_optimizer_finalize
	with flag argument.
	* tree-ssa-propagate.c (clean_up_loop_closed_phi): New function.

gcc/testsuite/ChangeLog:
2020-10-18  Jiufu Guo   <guojiufu@linux.ibm.com>

	* gcc.dg/tree-ssa/loopclosedphi.c: New test.