Show "ptr" for pointer use in cross-reference list

Consider:
 LDA $00 loads a value from address $00
 LDA $00,X might load from $00, or might not
 LDA ($00),Y dereferences $00 as a 16-bit pointer
 LDA ($00,X) dereferences a pointer, not necessarily from $00

When perusing the cross-reference list, it's useful to be able to
tell whether an instruction is accessing the location, using it as a
base address, or deferencing it as a pointer.  We now show "ptr" in
the list for pointer dereferences.  (We already showed "idx" for
indexed accesses.)

Asm65/OpDef.cs

@@ -253,6 +253,23 @@ namespace Asm65 {
             }
         }
 
+        /// <summary>
+        /// True for instructions that dereference the operand.
+        /// </summary>
+        public bool IsPointerAccessInstruction {
+            get {
+                return AddrMode == AddressMode.DPIndexXInd ||
+                       AddrMode == AddressMode.DPInd ||
+                       AddrMode == AddressMode.DPIndLong ||
+                       AddrMode == AddressMode.DPIndIndexY ||
+                       AddrMode == AddressMode.DPIndIndexYLong ||
+                       // indirect JMP/JSR
+                       AddrMode == AddressMode.AbsInd ||
+                       AddrMode == AddressMode.AbsIndLong ||
+                       AddrMode == AddressMode.AbsIndexXInd;
+            }
+        }
+
         /// <summary>
         /// True if this is an absolute-address instruction whose operand is combined with
         /// the Program Bank Register.