[release-branch.go1.9] runtime: make runtime.GC() trigger GC even if GOGC=off

Currently, the priority of checks in (gcTrigger).test() puts the
gcpercent<0 test above gcTriggerCycle, which is used for runtime.GC().
This is an unintentional change from 1.8 and before, where
runtime.GC() triggered a GC even if GOGC=off.

Fix this by rearranging the priority so the gcTriggerCycle test
executes even if gcpercent < 0.

Fixes #22023.

Change-Id: I109328d7b643b6824eb9d79061a9e775f0149575
Reviewed-on: https://go-review.googlesource.com/65994


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-on: https://go-review.googlesource.com/70979


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>