Known DXBC quirks

[doitsujin]

Just putting this here so it's not forgotten about. The following potential issues are being addressed in DXVK and also exist here:

• Vertex positions are assumed to be invariant, but FXC does not generate consistent code (mad vs mul + add even for identical subexpressions), so we need to explicitly emit mul + add to get consistent code gen and avoid issues like SotTR, for which we currently have an app profile.
• Precise mad is only emitted for the HLSL mad intrinsic in a precise context, so we should still be able to emit FFma for that.
• Even non-precise dpX instructions appear to get treated as a unit on native D3D drivers and are not further transformed even if there would be spec-compliant ways for doing so. We haven't seen many games rely on this besides Some water in Trails through Daybreak renders incorrectly on Nvidia GPU's doitsujin/dxvk#4162.