The 17-31 byte code is broken.  Disabled it.

Added a bunch of tests to at least cover the cases
in indexShortStr.  I'll channel Brad and wonder why
this CL ever got in without any tests.

Fixes #15679

Change-Id: I84a7b283a74107db865b9586c955dcf5f2d60161
Reviewed-on: https://go-review.googlesource.com/23106Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>