Better error message for shift context types. Fix shift of floating point constant to generate constant.
Better error message for shift context types.
Fix shift of floating point constant to generate constant.

R=iant
CC=gofrontend-dev
https://golang.org/cl/5089047

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