unicode/utf8: optimize Valid and ValidString for ASCII checks

Add a fastpath that uses 32bit loads and compares to check
8 ASCII characters per loop iteration.

This avoids the overhead of comparing and branching
for every byte individually.

Combining two 32bit loads into an uint32 allows the same
code to be used for 32bit and 64bit platforms.

amd64 (Intel i7-3520M):
name                         old time/op  new time/op  delta
ValidTenASCIIChars           15.6ns ± 4%   8.5ns ±14%  -45.27%  (p=0.000 n=10+10)
ValidTenJapaneseChars        50.0ns ± 2%  52.7ns ±15%     ~     (p=0.469 n=10+10)
ValidStringTenASCIIChars     13.5ns ± 1%   7.9ns ± 5%  -41.56%  (p=0.000 n=10+10)
ValidStringTenJapaneseChars  46.3ns ± 2%  45.8ns ± 2%     ~     (p=0.085 n=10+10)

arm (Raspberry Pi 3):
name                         old time/op  new time/op  delta
ValidTenASCIIChars           87.5ns ± 0%  58.5ns ± 0%  -33.11%  (p=0.000 n=9+10)
ValidTenJapaneseChars         359ns ± 0%   384ns ± 0%   +6.96%  (p=0.000 n=10+9)
ValidStringTenASCIIChars     87.5ns ± 0%  57.5ns ± 0%  -34.31%  (p=0.000 n=10+10)
ValidStringTenJapaneseChars   356ns ± 0%   377ns ± 0%   +5.90%  (p=0.000 n=10+10)

Change-Id: I9da942bddb250ee1f0ef7aabb4a8cb48edd9053e
Reviewed-on: https://go-review.googlesource.com/c/go/+/228823


Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>