* src/pic16/pcode.c: added helpers defmapInsertAfter (insert a new item

* src/pic16/pcode.c: added helpers defmapInsertAfter (insert a new item
  into the chain after a given one) and mergeDefmapSymbols (combine
  defmap entries for each symbol per pcode)
* (createDefmap): have defmap entries merged in the end
* (defmapReplaceSymRef): split defmap entries covering two accesses to
  a symbol before replacing one access type's symbol, merge symbols in
  the end (replacement symbol might already have an entry)
* (assignValnums): keep defmap entry for written WREG intact

git-svn-id: https://sdcc.svn.sourceforge.net/svnroot/sdcc/trunk/sdcc@3728 4a8a32a2-be11-0410-ad9d-d568d2c75423