Optimize AttributeBuffer to OutputVertex conversion (#3283)

Optimize AttributeBuffer to OutputVertex conversion

First I unrolled the inner loop, then I pushed semantics validation
outside of the hotloop.

I also added overflow slots to avoid conditional branches.

Super Mario 3D Land's intro runs at almost full speed when compiled with
Clang, and theres a noticible speed increase in MSVC. GCC hasn't been
tested but I'm confident in its ability to optimize this code.

src/video_core/shader/shader.h

@@ -50,6 +50,7 @@ struct OutputVertex {
     INSERT_PADDING_WORDS(1);
     Math::Vec2<float24> tc2;
 
+    static void ValidateSemantics(const RasterizerRegs& regs);
     static OutputVertex FromAttributeBuffer(const RasterizerRegs& regs,
                                             const AttributeBuffer& output);
 };