PHIOPT: Move tree_ssa_cs_elim into pass_cselim::execute.

PHIOPT: Move tree_ssa_cs_elim into pass_cselim::execute.

This moves around the code for tree_ssa_cs_elim slightly
improving code readability and removing declarations that
are no longer needed.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (tree_ssa_phiopt_worker): Remove declaration.
	(make_pass_phiopt): Make execute out of line.
	(tree_ssa_cs_elim): Move code into ...
	(pass_cselim::execute): here.