This simplifies covering all cases, reducing the number of branches
and making unrolling for simpler functions manageable.
This significantly improves performance of non-ASCII input.

This change will also allow addressing Issue #11733 in an efficient
manner.

RuneCountTenASCIIChars-8             13.7ns ± 4%  13.5ns ± 2%     ~     (p=0.116 n=7+8)
RuneCountTenJapaneseChars-8           153ns ± 3%    74ns ± 2%  -51.42%  (p=0.000 n=8+8)
RuneCountInStringTenASCIIChars-8     13.5ns ± 2%  12.5ns ± 3%   -7.13%  (p=0.000 n=8+7)
RuneCountInStringTenJapaneseChars-8   145ns ± 2%    68ns ± 2%  -53.21%  (p=0.000 n=8+8)
ValidTenASCIIChars-8                 14.1ns ± 3%  12.5ns ± 5%  -11.38%  (p=0.000 n=8+8)
ValidTenJapaneseChars-8               147ns ± 3%    71ns ± 4%  -51.72%  (p=0.000 n=8+8)
ValidStringTenASCIIChars-8           12.5ns ± 3%  12.3ns ± 3%     ~     (p=0.095 n=8+8)
ValidStringTenJapaneseChars-8         146ns ± 4%    70ns ± 2%  -51.62%  (p=0.000 n=8+7)
DecodeASCIIRune-8                    5.91ns ± 2%  4.83ns ± 3%  -18.28%  (p=0.001 n=7+7)
DecodeJapaneseRune-8                 12.2ns ± 7%   8.5ns ± 3%  -29.79%  (p=0.000 n=8+7)
FullASCIIRune-8                      5.95ns ± 3%  4.27ns ± 1%  -28.23%  (p=0.000 n=8+7)
FullJapaneseRune-8                   12.0ns ± 6%   4.3ns ± 3%  -64.39%  (p=0.000 n=8+8)

Change-Id: Iea1d6b0180cbbee1739659a0a38038126beecaca
Reviewed-on: https://go-review.googlesource.com/16940Reviewed-by: Russ Cox <rsc@golang.org>