ira: Add comments and fix move_spill_restore calculation

ira: Add comments and fix move_spill_restore calculation

This patch adds comments to describe each use of ira_loop_border_costs.
I think this highlights that move_spill_restore was using the wrong cost
in one case, which came from tranposing [0] and [1] in the original
(pre-ira_loop_border_costs) ira_memory_move_cost expressions.  The
difference would only be noticeable on targets that distinguish between
load and store costs.

gcc/
	PR rtl-optimization/98782
	* ira-color.c (color_pass): Add comments to describe the spill costs.
	(move_spill_restore): Likewise.  Fix reversed calculation.