Don't crash lowering self-referential variable initializer.
Don't crash lowering self-referential variable initializer.

R=iant
CC=gofrontend-dev
https://golang.org/cl/4171052

anlhord/gofrontend,golang/gofrontend,golang/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,golang/gofrontend,golang/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend