tree-object-size: Replace magic numbers with enums

A simple cleanup to allow inserting dynamic size code more easily.

gcc/ChangeLog:

	* tree-object-size.c: New enum.
	(object_sizes, computed, addr_object_size,
	compute_builtin_object_size, expr_object_size, call_object_size,
	merge_object_sizes, plus_stmt_object_size,
	collect_object_sizes_for, init_object_sizes, fini_object_sizes,
	object_sizes_execute): Replace magic numbers with enums.

Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>