combine: Correct comments about combine_validate_cost

combine: Correct comments about combine_validate_cost

Fix misleading comments.  That function only determines whether
replacements cost more; it doesn't actually *validate* costs as being
cheaper.

For example, it returns true also if it for various reasons cannot
determine the costs, or if the new cost is the same, like when doing
an identity replacement.  The code has been the same since
r0-59417-g64b8935d4809f3.

	* combine.cc: Correct comments about combine_validate_cost.