compiler: No error if shift operand inherits interface type.
compiler: No error if shift operand inherits interface type.

R=iant
CC=gofrontend-dev
https://golang.org/cl/6543056

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