go/ssa: print dominator tree

This CL adds the immediate dominator to the information displayed
about each basic block in the control flow graph printed
by the ssadump -build=F flag.

Change-Id: I130458c7f698bed90f09a931ef3261ac5e7c0486
Reviewed-on: https://go-review.googlesource.com/c/tools/+/716840
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Markus Kusano <kusano@google.com>

Author: adonovan

go/ssa/builder_test.go

@@ -327,12 +327,12 @@ func init():
 0:                                                                entry P:0 S:2
 	t0 = *init$guard                                                   bool
 	if t0 goto 2 else 1
-1:                                                           init.start P:1 S:1
+1:                                                    init.start P:1 S:1 idom:0
 	*init$guard = true:bool
 	t1 = errors.init()                                                   ()
 	*i = 42:int
 	jump 2
-2:                                                            init.done P:2 S:0
+2:                                                     init.done P:2 S:0 idom:0
 	return
 
 `},

go/ssa/example_test.go

@@ -100,11 +100,11 @@ func Example_buildPackage() {
 	// 0:                                                                entry P:0 S:2
 	// 	t0 = *init$guard                                                   bool
 	// 	if t0 goto 2 else 1
-	// 1:                                                           init.start P:1 S:1
+	// 1:                                                    init.start P:1 S:1 idom:0
 	// 	*init$guard = true:bool
 	// 	t1 = fmt.init()                                                      ()
 	// 	jump 2
-	// 2:                                                            init.done P:2 S:0
+	// 2:                                                     init.done P:2 S:0 idom:0
 	// 	return
 	//
 	// # Name: hello.main

go/ssa/func.go

@@ -668,7 +668,11 @@ func WriteFunction(buf *bytes.Buffer, f *Function) {
 			continue
 		}
 		n, _ := fmt.Fprintf(buf, "%d:", b.Index)
+		// (|predecessors|, |successors|, immediate dominator)
 		bmsg := fmt.Sprintf("%s P:%d S:%d", b.Comment, len(b.Preds), len(b.Succs))
+		if b.Idom() != nil {
+			bmsg = fmt.Sprintf("%s idom:%d", bmsg, b.Idom().Index)
+		}
 		fmt.Fprintf(buf, "%*s%s\n", punchcard-1-n-len(bmsg), "", bmsg)
 
 		if false { // CFG debugging