var-tracking.c (struct variable_def): Added field refcount.

	PR/14362
	* var-tracking.c (struct variable_def): Added field refcount.
	(variable_htab_free): Decrease the refcount and delete variable
	only if there are no more references.
	(unshare_variable): New function.
	(vars_copy_1): Increase refcount instead of copying the variable.
	(variable_union): Share the variables where possible, unshare
	the variables if needed.
	(variable_different_p): Return false if var1 and var2 are
	the same structure.
	(variable_was_changed): Init the refcount of new variable.
	(set_frame_base_location): Unshare variable if needed.
	(set_variable_part): Init the refcount of new variable.
	Unshare the variables if needed.
	(delete_variable_part): Unshare the variables if needed.
	(emit_notes_for_differences_1): Init the refcount of new variable.
	(vt_add_function_parameters): Do not add function parameters to
	IN set of ENTRY_BLOCK_PTR because it is unused anyway.
	(vt_initialize): Do not add frame_base_decl to IN set of
	ENTRY_BLOCK_PTR because it is unused anyway.

From-SVN: r79307