math/bits: faster LeadingZeros and Len functions

math/bits: faster LeadingZeros and Len functions

benchmark                     old ns/op     new ns/op     delta
BenchmarkLeadingZeros-8       8.43          3.10          -63.23%
BenchmarkLeadingZeros8-8      8.13          1.33          -83.64%
BenchmarkLeadingZeros16-8     7.34          2.07          -71.80%
BenchmarkLeadingZeros32-8     7.99          2.87          -64.08%
BenchmarkLeadingZeros64-8     8.13          2.96          -63.59%

Measured on 2.3 GHz Intel Core i7 running macOS 10.12.3.

Change-Id: Id343531b408d42ac45f10c76f60e85bdb977f91e
Reviewed-on: https://go-review.googlesource.com/37582
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>