runtime: avoid using REP prefix for IndexByte

runtime: avoid using REP prefix for IndexByte

REP-prefixed instructions have a large startup cost.
Avoid them like the plague.

benchmark                  old ns/op     new ns/op     delta
BenchmarkIndexByte10-8     22.4          5.34          -76.16%

Fixes #13983

Change-Id: I857e956e240fc9681d053f2584ccf24c1b272bb3
Reviewed-on: https://go-review.googlesource.com/18703
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>