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>
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merge request