Dominator tree pre-order.

Add a DominatorTreePreorder data structure which can be initialized for
a DominatorTree and used for queries involving a pre-order of the
dominator tree.

Print out the pre-order and send it through filecheck in "test domtree"
file tests.

cranelift/filetests/domtree/basic.cton

@@ -16,3 +16,10 @@ function %test(i32) {
 ; sameln: ebb3
 ; sameln: ebb1
 ; sameln: ebb0
+
+; check: domtree_preorder {
+; nextln: ebb0: ebb1
+; nextln: ebb1: ebb3 ebb2
+; nextln: ebb3:
+; nextln: ebb2:
+; nextln: }