Merged requested to merge anholt/mesa:mul64-lower into main
NIR already has the necessary lowering, and the GLSL lowering violates GLSL IR validation rules. Once quadop lowering was turned off, the IR validation at the end of the compile path on DEBUG builds caught the problem.
In order to move the lowering to NIR, though, we need to make sure that drivers supporting these functions actually have the lowering flag set.
Fixes: #6461 (closed)
(alternative to !16420 (merged))