cmd/callgraph: make vta use internal version of CHA

That version is faster and uses less memory.

Change-Id: I1839aa672fab25833da2ffa2dfcb079db67dd922
Reviewed-on: https://go-review.googlesource.com/c/tools/+/610537
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

cmd/callgraph/main.go

diff --git a/cmd/callgraph/main.go b/cmd/callgraph/main.go
index 7853826..4443f17 100644
--- a/cmd/callgraph/main.go
+++ b/cmd/callgraph/main.go
@@ -226,7 +226,7 @@
 		// NB: RTA gives us Reachable and RuntimeTypes too.
 
 	case "vta":
-		cg = vta.CallGraph(ssautil.AllFunctions(prog), cha.CallGraph(prog))
+		cg = vta.CallGraph(ssautil.AllFunctions(prog), nil)
 
 	default:
 		return fmt.Errorf("unknown algorithm: %s", algo)