More tweaks to def sym editing

If you edit an existing symbol, the "is the label unique" test will
always false-positive match itself, so we have to explicitly handle
that case.  Dialogs like Edit Instruction Operand make things a bit
more complicated because they don't flush results to the symbol
table immediately, which means the symbol we pass into the Edit Def
Symbol dialog to edit isn't necessarily the one we need to exclude
from the label uniqueness test.

The dialog was using the initial value as both "original" and
"initial", which caused some issues.  We now pass both values in.

Also, removed some dead code identified by VS.

SourceGen/DataAnalysis.cs

@@ -1141,7 +1141,7 @@ namespace SourceGen {
         /// <param name="start">Offset of first byte in range.</param>
         /// <param name="end">Offset of last byte in range.</param>
         /// <param name="charTest">Character test delegate.</param>
-        /// <param name="limitHiBit">If set, the high bit in all character must be the
+        /// <param name="limitHiBit">If set, the high bit in all characters must be the
         ///   same.  Used to enforce a single encoding when "low or high ASCII" is used.</param>
         /// <returns>Number of strings found, or -1 if bad data identified.</returns>
         public static int RecognizeLen16Strings(byte[] fileData, int start, int end,