x/benchmarks: fix go vet warnings

Here are go vet warnings for this repository:

lang.org/x/benchmarks/driver
driver/driver.go:375:2: unreachable code
driver/driver.go:376:2: unreachable code
third_party/biogo-examples/igor/igor/cluster.go:51:9: github.com/biogo/store/interval.IntRange struct literal uses unkeyed fields
sweet/benchmarks/biogo-igor/igor.go:68:30: golang.org/x/benchmarks/third_party/biogo-examples/igor/igor.GroupConfig struct literal uses unkeyed fields

This patch will fix the above warnings.

Change-Id: Id36cc3fa951072d374b843eff9abd769874d3835
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/642895
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
3 files changed