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/regs_rasterizer.h

@@ -87,6 +87,8 @@ struct RasterizerRegs {
         BitField<8, 5, Semantic> map_y;
         BitField<16, 5, Semantic> map_z;
         BitField<24, 5, Semantic> map_w;
+
+        u32 raw;
     } vs_output_attributes[7];
 
     INSERT_PADDING_WORDS(0xe);