Move the shared code of IndexByte and IndexByteString into
indexbytebody. This will allow to implement optimizations (e.g.
for #29001) in a single function.

Change-Id: I1d550da8eb65f95e492a460a12058cc35b1162b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/167939
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>