go.tools/ssa/interp: rewrite initorder test.
go.tools/ssa/interp: rewrite initorder test.

Correct, this time.

R=gri
CC=golang-codereviews
https://codereview.appspot.com/52930043

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