Avoid gpr overwritting on Ld_C instruction (#371)

* Avoid gpr overwritting on LD_C instruction

* Address feedback

* Ignore invalid registers

Ryujinx.Graphics/Gal/Shader/ShaderIrOperGpr.cs

@@ -6,11 +6,18 @@ namespace Ryujinx.Graphics.Gal.Shader
 
         public bool IsConst => Index == ZRIndex;
 
+        public bool IsValidRegister => (Index <= ZRIndex);
+
         public int Index { get; set; }
 
         public ShaderIrOperGpr(int Index)
         {
             this.Index = Index;
         }
+
+        public static ShaderIrOperGpr MakeTemporary(int Index = 0)
+        {
+            return new ShaderIrOperGpr(0x100 + Index);
+        }
     }
 }