Historically the SPIR-V backend was only fed by the TPF parser,
which only generates _sat destination modifiers. Now it is fed
by the D3DBC parser too (among others), so it mustn't assert on
other modifiers.
Modifier _pp can be trivially ignored. Modifier _centroid would
probably require some handling, but I'm not immediately sure of
what should happen and it doesn't look like a very urgent thing
anyway, so I'm degrading the assertion to FIXME().
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