Douglas Wilson authored Nov 15, 2017 and Ben Gamari committed Nov 15, 2017

We were accumulating the gc times of the previous gc.
`stats.gc.{cpu,elappsed}_ns` were being accumulated into
`stats.gc_{cpu,elapsed}_ns` before they were set.

There is also a change in that heap profiling will no longer cause gc
events to
be emitted.

Reviewers: bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #14257, #14445

Differential Revision: https://phabricator.haskell.org/D4184