internal/bytealg: optimize IndexString on arm64

internal/bytealg: optimize IndexString on arm64

This CL adjusts the order of the branch instructions of the
code to make it easier for the LIKELY branch to happen.

Benchmarks:
name                            old time/op    new time/op    delta
pkg:strings goos:linux goarch:arm64
IndexHard2-8                      2.17ms ± 1%    1.23ms ± 0%  -43.34%  (p=0.008 n=5+5)
CountHard2-8                      2.13ms ± 1%    1.21ms ± 2%  -43.31%  (p=0.008 n=5+5)

pkg:bytes goos:linux goarch:arm64
IndexRune/4M-8                     661µs ±22%     513µs ± 0%  -22.32%  (p=0.008 n=5+5)
IndexEasy/4M-8                     672µs ±23%     513µs ± 0%  -23.71%  (p=0.016 n=5+4)

Change-Id: Ib96f095edf77747edc8a971e79f5c1428e5808ce
Reviewed-on: https://go-review.googlesource.com/109015
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>