nir: fix opt_if_loop_last_continue()

nir: fix opt_if_loop_last_continue()

The pass did not correctly handle loops ending in:

	if ssa_7 {
		block block_8:
		/* preds: block_7 */
		continue
		/* succs: block_1 */
	} else {
		block block_9:
		/* preds: block_7 */
		break
		/* succs: block_11 */
	}

The break will get eliminated by another opt but if this pass gets
called first (as it does on RADV) we ended up inserting
instructions after the break.

Fixes: 5921a19d4b0c ("nir: add if opt opt_if_loop_last_continue()")
Reviewed-by: Dave Airlie <airlied@redhat.com>