ira: Add a ira_loop_border_costs class

ira: Add a ira_loop_border_costs class

The final index into (ira_)memory_move_cost is 1 for loads and
0 for stores.  Thus the combination:

  entry_freq * memory_cost[1] + exit_freq * memory_cost[0]

is the cost of loading a register on entry to a loop and
storing it back on exit from the loop.  This is the cost to
use if the register is successfully allocated within the
loop but is spilled in the parent loop.  Similarly:

  entry_freq * memory_cost[0] + exit_freq * memory_cost[1]

is the cost of storing a register on entry to the loop and
restoring it on exit from the loop.  This is the cost to
use if the register is spilled within the loop but is
successfully allocated in the parent loop.

The patch adds a helper class for calculating these values and
mechanically replaces the existing instances.  There is no attempt to
editorialise the choice between using “spill inside” and “spill outside”
costs.  (I think one of them is the wrong way round, but a later patch
deals with that.)

No functional change intended.

gcc/
	PR rtl-optimization/98782
	* ira-int.h (ira_loop_border_costs): New class.
	* ira-color.c (ira_loop_border_costs::ira_loop_border_costs):
	New constructor.
	(calculate_allocno_spill_cost): Use ira_loop_border_costs.
	(color_pass): Likewise.
	(move_spill_restore): Likewise.