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: }

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: }

cranelift/filetests/domtree/loops2.cton

@@ -1,6 +1,6 @@
 test domtree
 
-function %test(i32) {
+function %loop1(i32) {
     ebb0(v0: i32):
         brz v0, ebb1    ; dominates: ebb1 ebb6
         brnz v0, ebb2   ; dominates: ebb2 ebb9
@@ -30,7 +30,20 @@ function %test(i32) {
         return
 }
 
-function %test(i32) native {
+; check: domtree_preorder {
+; nextln: ebb0: ebb1 ebb2 ebb6 ebb3 ebb9
+; nextln: ebb1:
+; nextln: ebb2: ebb4 ebb5 ebb7 ebb8
+; nextln: ebb4:
+; nextln: ebb5:
+; nextln: ebb7:
+; nextln: ebb8:
+; nextln: ebb6:
+; nextln: ebb3:
+; nextln: ebb9:
+; nextln: }
+
+function %loop2(i32) native {
     ebb0(v0: i32):
         brz v0, ebb1    ; dominates: ebb1 ebb3 ebb4 ebb5
         jump ebb2       ; dominates: ebb2
@@ -55,3 +68,12 @@ function %test(i32) native {
 ; sameln: ebb2
 ; 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: }

cranelift/filetests/domtree/tall-tree.cton

@@ -31,3 +31,18 @@ function %test(i32) {
     ebb11:
         return
 }
+
+; check: domtree_preorder {
+; nextln: ebb0: ebb1 ebb2 ebb3 ebb5
+; nextln: ebb1: ebb4
+; nextln: ebb4: ebb6 ebb7 ebb10
+; nextln: ebb6: ebb8 ebb9 ebb11
+; nextln: ebb8:
+; nextln: ebb9:
+; nextln: ebb11:
+; nextln: ebb7:
+; nextln: ebb10:
+; nextln: ebb2:
+; nextln: ebb3:
+; nextln: ebb5:
+; nextln: }

cranelift/filetests/domtree/wide-tree.cton

@@ -39,3 +39,20 @@ function %test(i32) {
     ebb13:
         return
 }
+
+; check: domtree_preorder {
+; nextln: ebb0: ebb13 ebb1
+; nextln: ebb13:
+; nextln: ebb1: ebb2 ebb3 ebb4 ebb5 ebb6 ebb7
+; nextln: ebb2:
+; nextln: ebb3:
+; nextln: ebb4:
+; nextln: ebb5:
+; nextln: ebb6:
+; nextln: ebb7: ebb8 ebb9 ebb10 ebb12 ebb11
+; nextln: ebb8:
+; nextln: ebb9:
+; nextln: ebb10:
+; nextln: ebb12:
+; nextln: ebb11:
+; nextln: }