* alias.c (get_alias_set): Try to replace PLACEHOLDER_EXPR.
	Loop through NOPs, placeholders, and components.
	Don't go through NOPs if change mode.
	(record_alias_subset): Do nothing if SUBSET and SET are the same.
	* emit-rtl.c (set_mem_alias_set): Enable check.
	* expr.c (find_placeholder): New function.
	(expand_expr, case PLACEHOLDER_EXPR): Use it.
	(expand_expr, case COMPONENT_EXPR): Always copy OP0 when we need
	to modify it and avoid unneeded copies.
	* expr.h (expand_expr): Always define.
	(find_placeholder): New declaration.

From-SVN: r45931