ira.c bb_loop_depth again

Follow the same practice as other places in ira.c, where
free_dominance_info is called along with loop_optimizer_finalize.  Not
doing so causes an ICE on gcc-5-branch, so avoid that possibility on
trunk.

	PR rtl-optimization/71275
	* ira.c (ira): Free dominance info.

From-SVN: r236843