spirv-val: Add missing TileImageEXT declared variable check

[spencer-lunarg]

VUID-StandaloneSpirv-None-08724

The VU says

The TileImageEXT Storage Class must only be used for declaring tile image variables

@janharaldfredriksen-arm this your extension, what does declaring tile image variables mean here? Is a chance you could have someone add this check?

[janharaldfredriksen-arm]

It means that the storage class can only be used for image type variables with Dim equal to TileImageDataEXT.