math/bits: added benchmarks for Leading/TrailingZeros
math/bits: added benchmarks for Leading/TrailingZeros

BenchmarkLeadingZeros-8      	200000000	         8.80 ns/op
BenchmarkLeadingZeros8-8     	200000000	         8.21 ns/op
BenchmarkLeadingZeros16-8    	200000000	         7.49 ns/op
BenchmarkLeadingZeros32-8    	200000000	         7.80 ns/op
BenchmarkLeadingZeros64-8    	200000000	         8.67 ns/op

BenchmarkTrailingZeros-8     	1000000000	         2.05 ns/op
BenchmarkTrailingZeros8-8    	2000000000	         1.94 ns/op
BenchmarkTrailingZeros16-8   	2000000000	         1.94 ns/op
BenchmarkTrailingZeros32-8   	2000000000	         1.92 ns/op
BenchmarkTrailingZeros64-8   	2000000000	         2.03 ns/op

Change-Id: I45497bf2d6369ba6cfc88ded05aa735908af8908
Reviewed-on: https://go-review.googlesource.com/37220
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

vsekhar/elastic-go,akutz/go,codestation/go,josharian/go,linux-on-ibm-z/go,Cofyc/go,akutz/go,CAFxX/go,nawawi/go,polyverse-security/go,Cofyc/go,polyverse-security/go,polyverse-security/go,akutz/go,linux-on-ibm-z/go,codestation/go,golang/go,CAFxX/go,codestation/go,josharian/go,golang/go,ImJasonH/go,ImJasonH/go,derekmarcotte/go,cloudflare/go,akutz/go,derekmarcotte/go,vsekhar/elastic-go,polyverse-security/go,polyverse-security/go,Cofyc/go,vsekhar/elastic-go,derekmarcotte/go,codestation/go,derekmarcotte/go,polyverse-security/go,linux-on-ibm-z/go,ImJasonH/go,Cofyc/go,linux-on-ibm-z/go,cloudflare/go,CAFxX/go,derekmarcotte/go,codestation/go,golang/go,ImJasonH/go,codestation/go,linux-on-ibm-z/go,cloudflare/go,vsekhar/elastic-go,nawawi/go,CAFxX/go,golang/go,ImJasonH/go,CAFxX/go,vsekhar/elastic-go,Cofyc/go,linux-on-ibm-z/go,Cofyc/go,linux-on-ibm-z/go,linux-on-ibm-z/go,cloudflare/go,ImJasonH/go,ImJasonH/go,cloudflare/go,josharian/go,derekmarcotte/go,derekmarcotte/go,golang/go,polyverse-security/go,cloudflare/go,vsekhar/elastic-go,josharian/go,ImJasonH/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,golang/go,nawawi/go,nawawi/go,akutz/go,nawawi/go,Cofyc/go,akutz/go,christopher-henderson/Go,polyverse-security/go,akutz/go,josharian/go,golang/go,josharian/go,josharian/go,cloudflare/go,CAFxX/go,vsekhar/elastic-go,Cofyc/go,codestation/go,codestation/go,vsekhar/elastic-go,nawawi/go,nawawi/go,derekmarcotte/go,akutz/go,nawawi/go