go.tools/go/ssa/interp: add missing case to fake reflect function.
go.tools/go/ssa/interp: add missing case to fake reflect function.

LGTM=gri
R=gri
CC=golang-codereviews
https://codereview.appspot.com/61410049

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