recog: Add a way of temporarily undoing changes

recog: Add a way of temporarily undoing changes

In some cases, it can be convenient to roll back the changes that
have been made by validate_change to see how things looked before,
then reroll the changes.  For example, this makes it possible
to defer calculating the cost of an instruction until we know that
the result is actually needed.  It can also make dumps easier to read.

This patch adds a couple of helper functions for doing that.

gcc/
	* recog.h (temporarily_undo_changes, redo_changes): Declare.
	* recog.c (temporarily_undone_changes): New variable.
	(validate_change_1, confirm_change_group): Check that it's zero.
	(cancel_changes): Likewise.
	(swap_change, temporarily_undo_changes): New functions.
	(redo_changes): Likewise.