PHIOPT: Split out store elimination from phiopt

PHIOPT: Split out store elimination from phiopt

Since the last cleanups, it made easier to see
that we should split out the store elimination
worker from tree_ssa_phiopt_worker function.

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

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (tree_ssa_phiopt_worker): Remove
	do_store_elim argument and split that part out to ...
	(store_elim_worker): This new function.
	(pass_cselim::execute): Call store_elim_worker.
	(pass_phiopt::execute): Update call to tree_ssa_phiopt_worker.