vkd3d-shader/spirv: Handle all possible destination modifiers.

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().