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 <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: zpavlinovic

cmd/callgraph/main.go

@@ -226,7 +226,7 @@ func doCallgraph(dir, gopath, algo, format string, tests bool, args []string) er
 		// 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)