go.tool.pointer: fix regression in pointer.cgraph.Root().

The previous CL made the assumption that Root is the first
node, which is false for programs that import "reflect".
Reverted to the previous way: an explicit root field.

Added regression test (callgraph2) via oracle.

R=crawshaw
TBR=crawshaw
CC=golang-dev
https://golang.org/cl/13967043

oracle/oracle_test.go

@@ -200,6 +200,7 @@ func TestOracle(t *testing.T) {
 	for _, filename := range []string{
 		"testdata/src/main/calls.go",
 		"testdata/src/main/callgraph.go",
+		"testdata/src/main/callgraph2.go",
 		"testdata/src/main/describe.go",
 		"testdata/src/main/freevars.go",
 		"testdata/src/main/implements.go",

oracle/testdata/src/main/callgraph2.go

@@ -0,0 +1,14 @@
+package main
+
+// Tests of call-graph queries.
+// See go.tools/oracle/oracle_test.go for explanation.
+// See callgraph2.golden for expected query results.
+
+import _ "reflect"
+
+func f() {}
+func main() {
+	f()
+}
+
+// @callgraph callgraph "^"

oracle/testdata/src/main/callgraph2.golden

@@ -0,0 +1,15 @@
+-------- @callgraph callgraph --------
+
+Below is a call graph of the entire program.
+The numbered nodes form a spanning tree.
+Non-numbered nodes indicate back- or cross-edges to the node whose
+ number follows in parentheses.
+Some nodes may appear multiple times due to context-sensitive
+ treatment of some calls.
+
+0	<root>
+1	    main.init
+2	        reflect.init
+3	    main.main
+4	        main.f
+

pointer/analysis.go

@@ -267,7 +267,7 @@ func Analyze(config *Config) *Result {
 		fmt.Fprintln(a.log, "======== NEW ANALYSIS ========")
 	}
 
-	a.generate()
+	root := a.generate()
 
 	//a.optimize()
 
@@ -304,7 +304,7 @@ func Analyze(config *Config) *Result {
 
 	var callgraph *cgraph
 	if a.config.BuildCallGraph {
-		callgraph = &cgraph{a.cgnodes}
+		callgraph = &cgraph{root, a.cgnodes}
 	}
 
 	return &Result{

pointer/callgraph.go

@@ -16,6 +16,7 @@ import (
 
 // cgraph implements call.Graph.
 type cgraph struct {
+	root  *cgnode
 	nodes []*cgnode
 }
 
@@ -28,7 +29,7 @@ func (g *cgraph) Nodes() []call.GraphNode {
 }
 
 func (g *cgraph) Root() call.GraphNode {
-	return g.nodes[0]
+	return g.root
 }
 
 // cgnode implements call.GraphNode.