go.tools/go/types: minor internal cleanup
go.tools/go/types: minor internal cleanup

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/80960046

d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,8limbs/gotools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,d0f/go-zh.tools,8limbs/gotools,skyportsystems/go.tools,skyportsystems/go.tools,d0f/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools