Bug 1526676 - demote SM(msan) back to tier 3; it is getting run via ridealongs, r=catlee

Differential Revision: https://phabricator.services.mozilla.com/D19698

--HG--
extra : moz-landing-system : lando

taskcluster/ci/spidermonkey/linux.yml

@@ -126,6 +126,7 @@ sm-msan-linux64/opt:
     treeherder:
         symbol: SM(msan)
         platform: linux64/opt
+        tier: 3
     run:
         spidermonkey-variant: msan
     # Disable by default by allow try pushes to explicitly request.