spirv: Add helper to find args of Image Operands
Avoid keeping track of the idx and all possible image operands for each operation. Note for convenience we split up the handling of ImageOperandsOffsetMask and ImageOperandsConstOffsetMask. Suggested by Jason Ekstrand. Reviewed-by: Jason Ekstrand <jason@jlekstrand.net> Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>