[dev.garbage] cmd/gc: never generate BitsMultiWord
[dev.garbage] cmd/gc: never generate BitsMultiWord

LGTM=rlh
R=rlh, minux
CC=golang-codereviews
https://codereview.appspot.com/151940043

d0f/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh