compiler: Don't crash on invalid arithmetic ops.
compiler: Don't crash on invalid arithmetic ops.

The gofrontend would crash after hitting an unreachable state while
trying to determine the type of an arithmetic expression involving
non-numeric values.  Instead of crashing, it should fail gracefully
if the relevant error is already reported.

Fixes golang/go#11537.

Change-Id: I84ec06bb944b8af34ec5e5864a5149c11c782919
Reviewed-on: https://go-review.googlesource.com/13793
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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