Implement fixed-point variant of the UCVTF and SCVTF instructions (#578)

* Add fixed-point variant of the UCVTF instruction

* Change encoding of some fixed-point instructions to not allow invalid encodings

* Fix Fcvtzu_Gp_Fixed encoding

* Add SCVTF (fixed-point GP to Scalar) instruction

* Simplify *Fixed encodings

ChocolArm64/Decoders/OpCodeSimdCvt64.cs

@@ -8,18 +8,9 @@ namespace ChocolArm64.Decoders
 
         public OpCodeSimdCvt64(Inst inst, long position, int opCode) : base(inst, position, opCode)
         {
-            //TODO:
-            //Und of Fixed Point variants.
             int scale = (opCode >> 10) & 0x3f;
             int sf    = (opCode >> 31) & 0x1;
 
-            /*if (Type != SF && !(Type == 2 && SF == 1))
-            {
-                Emitter = AInstEmit.Und;
-
-                return;
-            }*/
-
             FBits = 64 - scale;
 
             RegisterSize = sf != 0