Add targetm.insn_cost hook

Add targetm.insn_cost hook

This adds a new hook that the insn_cost function uses if a target has
implemented it (it uses the old pattern_cost nee insn_rtx_cost if not).

	* target.def (insn_cost): New hook.
	* doc/tm.texi.in (TARGET_INSN_COST): New hook.
	* doc/tm.texi: Regenerate.
	* rtlanal.c (insn_cost): Use the new hook.

From-SVN: r253562