radv: Call nir_propagate_invariant()

Without this, invariant qualifiers don't do anything. Together with a
fix to the game, this fixes flickering in No Man's Sky.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
22 jobs for !1880 with radv-invariant-fix in 18 minutes and 29 seconds (queued for 6 seconds)
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