compiler: Use backend interface for type conversions.
compiler: Use backend interface for type conversions.

R=iant
CC=gofrontend-dev
https://golang.org/cl/71230046

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