This simplifies _Traits_find_first_not_of and _Traits_find_last_not_of, following the patterns in #4744 and #4934, which will make them easier to understand and easier to add vectorized codepaths. These are lossless transformations that don't affect behavior.

• Replace unnecessarily confusing breaks with immediate returns.
  • Except in _Traits_find_last_of, which is being changed by Vectorize basic_string::find_last_of #4934.
• Add a return instead of relying on fall-through.
  • This will simplify the following transformation.
• Flip !_Matches._Mark tests.
  • Both sides unconditionally return, allowing us to flip them. (One case is a forever loop where the only ways out are returns.)
• Transform "call self with _Special=false" to fall-through.
• Extract _Elem.
• Extract _Hay_start, _Hay_end.
• Fuse bool _Special into its usage.