PHIOPT: Rename tree_ssa_phiopt_worker to pass_phiopt::execute

PHIOPT: Rename tree_ssa_phiopt_worker to pass_phiopt::execute

Now that store elimination and phiopt does not
share outer code, we can move tree_ssa_phiopt_worker
directly into pass_phiopt::execute and remove
many declarations (prototypes) from the file.

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (two_value_replacement): Remove
	prototype.
	(match_simplify_replacement): Likewise.
	(factor_out_conditional_conversion): Likewise.
	(value_replacement): Likewise.
	(minmax_replacement): Likewise.
	(spaceship_replacement): Likewise.
	(cond_removal_in_builtin_zero_pattern): Likewise.
	(hoist_adjacent_loads): Likewise.
	(tree_ssa_phiopt_worker): Move into ...
	(pass_phiopt::execute): this.