While adding geometry stage support for clip/cull, it transpired that the
existing clip/cull support was not setting the type of the result of indexing
into the clup/cull variable.  That's a defect independent of the geometry
support, so to simplify the geometry PR, this is addressed separately.

It doesn't appear to change the generated SPIR-V, but that's probably down to
something else tolerating a bad input.