tree-predcom.c: Add general comment on Store-Store chains.

tree-predcom.c: Add general comment on Store-Store chains.

	* tree-predcom.c: Add general comment on Store-Store chains.
	(split_data_refs_to_components): Postpone clearing eliminate_store_p
	flag in component.
	(get_chain_last_ref_at): Rename into...
	(get_chain_last_write_at): ...this.
	(get_chain_last_write_before_load): New function.
	(add_ref_to_chain): Promote type of chain from CT_STORE_LOAD to
	CT_STORE_STORE when write reference is added.
	(determine_roots_comp): Support load ref in CT_STORE_STORE chains.
	(is_inv_store_elimination_chain): Update get_chain_last_write_at call.
	(initialize_root_vars_store_elim_1): Ditto.
	(initialize_root_vars_store_elim_2): Ditto.  Replace rhs once default
	definition is created.
	(execute_pred_commoning_chain): Support load ref in CT_STORE_STORE
	chain by replacing it with dominant stored value.

	gcc/testsuite
	* gcc.dg/tree-ssa/predcom-dse-12.c: New test.

From-SVN: r254956