regexp/syntax: optimize EmptyOpContext

Minor. Saw this in a profile at few percent of CPU and was
curious what it was. Improves overall regexp benchmarks
anywhere from 0 to 3%, but they're a pain to run. You need to
run them in isolation for long runs to get stable numbers.

benchmark                  old ns/op    new ns/op    delta
BenchmarkEmptyOpContext          537          473  -11.92%

R=golang-dev, crawshaw
CC=golang-dev
https://codereview.appspot.com/13407043