compilebench emits both ns/op and user-ns/op measurements.
This CL improves benchstat's handling of such measurements.
Migrated from https://github.com/rsc/benchstat/pull/5.

Sample output, before:

name       old user-ns/op  new user-ns/op  delta
Template        279M ± 7%       273M ± 4%     ~     (p=0.329 n=6+5)
Unicode         148M ±11%       131M ± 7%  -11.49%  (p=0.030 n=6+5)
GoTypes         830M ± 2%       835M ± 3%     ~     (p=0.792 n=6+5)
SSA            9.60G ± 3%      9.64G ± 2%     ~     (p=0.792 n=6+5)
Flate           158M ± 3%       155M ± 1%     ~     (p=0.329 n=6+5)
GoParser        204M ± 6%       202M ± 8%     ~     (p=0.792 n=6+5)
Reflect         511M ± 9%       531M ± 6%     ~     (p=0.177 n=6+5)
Tar             151M ± 3%       151M ± 5%     ~     (p=0.931 n=6+5)
XML             271M ± 3%       268M ± 3%     ~     (p=0.429 n=6+5)

Sample output, after:

name       old user-time/op  new user-time/op  delta
Template         279ms ± 7%        273ms ± 4%     ~     (p=0.329 n=6+5)
Unicode          148ms ±11%        131ms ± 7%  -11.49%  (p=0.030 n=6+5)
GoTypes          830ms ± 2%        835ms ± 3%     ~     (p=0.792 n=6+5)
SSA              9.60s ± 3%        9.64s ± 2%     ~     (p=0.792 n=6+5)
Flate            158ms ± 3%        155ms ± 1%     ~     (p=0.329 n=6+5)
GoParser         204ms ± 6%        202ms ± 8%     ~     (p=0.792 n=6+5)
Reflect          511ms ± 9%        531ms ± 6%     ~     (p=0.177 n=6+5)
Tar              151ms ± 3%        151ms ± 5%     ~     (p=0.931 n=6+5)
XML              271ms ± 3%        268ms ± 3%     ~     (p=0.429 n=6+5)

Change-Id: I716b4e89a28adc72e9135b8c580434f748a9c0a9
Reviewed-on: https://go-review.googlesource.com/39794
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>