compiler: Report errors for non-integral shift counts.
compiler: Report errors for non-integral shift counts.

Fixes golang/go#12618.

Change-Id: Ifb069bddad43f11ff2437a186b050d80a21e1c28
Reviewed-on: https://go-review.googlesource.com/14647
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend