go.tools: use golang.org/x/... import paths

Rewrite performed with this command:
  sed -i '' 's_code.google.com/p/go\._golang.org/x/_g' \
    $(grep -lr 'code.google.com/p/go.' *)

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/170920043
diff --git a/go/ssa/ssa.go b/go/ssa/ssa.go
index 09fa75f..8543992 100644
--- a/go/ssa/ssa.go
+++ b/go/ssa/ssa.go
@@ -13,10 +13,10 @@
 	"go/token"
 	"sync"
 
-	"code.google.com/p/go.tools/go/exact"
-	"code.google.com/p/go.tools/go/loader"
-	"code.google.com/p/go.tools/go/types"
-	"code.google.com/p/go.tools/go/types/typeutil"
+	"golang.org/x/tools/go/exact"
+	"golang.org/x/tools/go/loader"
+	"golang.org/x/tools/go/types"
+	"golang.org/x/tools/go/types/typeutil"
 )
 
 // A Program is a partial or complete Go program converted to SSA form.