go.tools/ssa/interp: fix build
go.tools/ssa/interp: fix build

fallthrough's are now checked - adjust
now illegal test.

TBR: adonovan

R=adonovan
CC=golang-dev
https://codereview.appspot.com/12041052

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