[arm] Correct cost calculations involving borrow for subtracts.

[arm] Correct cost calculations involving borrow for subtracts.

The rtx_cost calculations when a borrow operation was being performed were
not being calculated correctly.  The borrow is free as part of the
subtract-with-carry instructions.  This patch recognizes the various
idioms that can describe this and returns the correct costs.

	* config/arm/arm.c (arm_rtx_costs_internal, case MINUS): Handle
	borrow operations.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@277173 138bc75d-0d04-0410-961f-82ee72b054a4