For non-unit increment, loopbce checks to see if the
increment evenly divides the difference between (constant)
loop start and end.  This test panics when the increment
is zero.

Fix: check for zero, if found, don't optimize the loop.

Also added missing copyright notice to loopbce.go.

Fixes #26043.

Change-Id: I5f460104879cacc94481949234c9ce8c519d6380
Reviewed-on: https://go-review.googlesource.com/120759
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>