Bug 1626496 - fix bit casting of vectors for GCC. r=jrmuizel

Depends on D69150

Differential Revision: https://phabricator.services.mozilla.com/D69151

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/929cb28d1a1f98158e675a3390669228cdbcc57e

Lee Salzman authored and moz-gfx committed Apr 1, 2020