One (12466) was an actual logic error, backing up when there was
nothing there. The others were due to continuing to process an
instruction when it cannot work.
Methodically stop assembling an instruction when it's not going to
succeed.
Fixes #12466.
Fixes #12467.
Fixes #12468.
Change-Id: I88c568f2b9c1a8408043b2ac5a78f5e2ffd62abd
Reviewed-on: https://go-review.googlesource.com/14498 Reviewed-by: Andrew Gerrand <adg@golang.org>