DXIL doesn't support a single varying having components belonging
to multiple streams. For geometry shader outputs, whenever we find
this to be the case, break up that variable into multiple sub-
variables, and update stores to that variable to write to the new
sub-variables instead.
Reviewed-by: Sil Vilerino <sivileri@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14787>
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