haifa-sched exports an insn_cost function, but it is only used in a
few places and specialised to scheduling.  This patch renames it to
insn_sched_cost.


	* haifa-sched.c: Rename insn_cost to insn_sched_cost.
	* sched-rgn.c: Ditto.
	* sel-sched-ir.c: Ditto.

From-SVN: r253072