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/loops.cton

@@ -50,6 +50,15 @@ function %test(i32) {
 ; sameln: ebb1
 ; sameln: ebb0
 
+; check: domtree_preorder {
+; nextln: ebb0: ebb1 ebb2 ebb4 ebb3 ebb5
+; nextln: ebb1:
+; nextln: ebb2:
+; nextln: ebb4:
+; nextln: ebb3:
+; nextln: ebb5:
+; nextln: }
+
 function %loop2(i32) native {
     ebb0(v0: i32):
         brz v0, ebb1    ; dominates: ebb1 ebb3 ebb4 ebb5
@@ -82,3 +91,14 @@ function %loop2(i32) native {
 ; sameln: ebb2
 ; sameln: ebb1
 ; sameln: ebb0
+
+; check: domtree_preorder {
+; nextln: ebb0: ebb1 ebb2 ebb4 ebb3 ebb5
+; nextln: ebb1:
+; nextln: ebb2:
+; nextln: ebb4: ebb6
+; nextln: ebb6: ebb7
+; nextln: ebb7:
+; nextln: ebb3:
+; nextln: ebb5:
+; nextln: }