Initial support for double precision shader instructions. (#963)

* Implement DADD, DFMA and DMUL shader instructions

* Rename FP to FP32

* Correct double immediate

* Classic mistake

Ryujinx.Graphics.Shader/Decoders/FPType.cs

@@ -1,3 +1,5 @@
+using Ryujinx.Graphics.Shader.IntermediateRepresentation;
+
 namespace Ryujinx.Graphics.Shader.Decoders
 {
     enum FPType
@@ -6,4 +8,12 @@ namespace Ryujinx.Graphics.Shader.Decoders
         FP32 = 2,
         FP64 = 3
     }
+
+    static class FPTypeExtensions
+    {
+        public static Instruction ToInstFPType(this FPType type)
+        {
+            return type == FPType.FP64 ? Instruction.FP64 : Instruction.FP32;
+        }
+    }
 }