regexp: fix BenchmarkMatch_onepass_regex
regexp: fix BenchmarkMatch_onepass_regex

This benchmark - in contrast to all other benchmarks - was
running the regexp match on 1-byte substrings of the input
instead of the entire input. Worse, it was doing so by preallocating
a slice of slices of every 1-byte substring. Needless to say,
this does not accurately reflect what happens when the regexp
matcher is given a large input.

Change-Id: Icd5b95f0e43f554a6b93164916745941366e03d6
Reviewed-on: https://go-review.googlesource.com/c/139778
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,cloudflare/go,josharian/go,josharian/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,akutz/go,cloudflare/go,golang/go,josharian/go,nawawi/go,cloudflare/go,akutz/go,nawawi/go,CAFxX/go,golang/go,akutz/go,golang/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,akutz/go,nawawi/go,nawawi/go,josharian/go,golang/go,josharian/go,CAFxX/go,akutz/go,josharian/go,akutz/go,golang/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,josharian/go