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

t-yuki/go.tools.old,tmc/mirror-go.tools,tmc/mirror-go.tools,t-yuki/go.tools.old,t-yuki/go.tools.old,tmc/mirror-go.tools