Re-writing the switch statement as a single boolean expression
reduces the number of branches that the compiler generates.
It is also arguably easier to read as a pair of numeric ranges
that valid runes can exist in.
No test changes since the existing test does a good job of
testing all of the boundaries.
This change was to gain back some performance after a correctness
fix done in http://golang.org/cl/32123.
The correctness fix (CL/32123) slowed down the benchmarks slightly:
benchmark old ns/op new ns/op delta
BenchmarkIndexRune/10-4 19.3 21.6 +11.92%
BenchmarkIndexRune/32-4 33.6 35.2 +4.76%
Since the fix relies on utf8.ValidRune, this CL improves benchmarks:
benchmark old ns/op new ns/op delta
BenchmarkIndexRune/10-4 21.6 20.0 -7.41%
BenchmarkIndexRune/32-4 35.2 33.5 -4.83%