Thor mentioned in the film itself that Thanos killed half of his people only:
Thor: There's six stones out there. Thanos already has the Power Stone because he stole it last week, when he decimated Xandar. He stole the Space Stone from me, when he destroyed my ship and slaughtered half my people. The Time and Mind Stones are safe on Earth. They're with the Avengers. source
And From comicbook.com:
According to the Redditor, Russo confirmed Valkyrie did survive Thanos’ ambush of the Sanctuary II. The director did not say how, but he did say a portion of the Asgardians also survived. Russo said escape pods ferried those survivors away from the battle, giving the Asgardian race the chance to escape extinction. It is very possible the Valkyrie was charged with overseeing that evacuation since most of the Asgardian refugees were simple civilians.
So she survived and is roaming around somewhere.
Or maybe she just became a goat ;p
Update from Avengers: Endgame-
Valkyrie is alive and well in New Asgard after the event of Avengers: Infinity War. She even participates in the final showdown with other female heroes on a winged horse after Hulk's snap.