Whenever one of them is BRW_SOMETIMES, we depend on dynamic flag pushed
in as a push constant. In this case, we have to often have to do the
calculation both ways and SEL the result. It's a bit more code but
decouples MSAA from the shader key.
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21094>
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