Fix add-label shortcut for adjusted operands

When you edit the operand of an instruction that targets an in-file
address, you're given the opportunity to specify a shortcut that
applies the symbol to the instruction's target address in addition
to or instead of defining a weak symbol reference on the instruction
being edited.

This didn't work right for operands with adjustments, e.g. the store
instructions in self-modifying code.  It put the label at the
unadjusted offset, which does nothing useful.

We now correctly back up to the start of the instruction or multi-
byte data area.

SourceGen/DataAnalysis.cs

@@ -229,6 +229,25 @@ namespace SourceGen {
             return operandOffset;
         }
 
+        /// <summary>
+        /// Returns the "base" operand offset.  If the byte at the specified offset is not the
+        /// start of a code/data/inline-data item, walk backward until the start is found.
+        /// </summary>
+        /// <param name="proj">Project reference.</param>
+        /// <param name="offset">Start offset.</param>
+        /// <returns></returns>
+        public static int GetBaseOperandOffset(DisasmProject proj, int offset) {
+            Debug.Assert(offset >= 0 && offset < proj.FileDataLength);
+            while (!proj.GetAnattrib(offset).IsStart) {
+                offset--;
+
+                // Should not be possible to walk off the top of the list, since we're in
+                // the middle of something.
+                Debug.Assert(offset >= 0);
+            }
+            return offset;
+        }
+
         /// <summary>
         /// Creates a FormatDescriptor in the Anattrib array at srcOffset that links to
         /// targetOffset, or a nearby label.  If targetOffset doesn't have a useful label,