op_private: Remove incorrect OPpLVREF_TYPE comment

At the time, I was trying to rearrange the magic flags so that
OPpLVREF_TYPE could be copied into mg->mg_flags for lvref magic.  Then
I saw that mg_private was available and gave up on the whole idea; but
this comment remained.
Renumber op flags so TARGET_MY and OUR_INTRO differ

I need split to be able to take both flags.  Renumbering OUR_INTRO
sets off a chain reaction requiring that various other flags be renum-
bered.  These are the affected ops:

      gvsv  rv2sv rv2av rv2hv  enteriter split rv2gv padsv aelem helem entersub padav padhv lvavref lvref refassign pushmark rv2cv
----------------------------------------------------------------------------------------------------------------------------------
0                                                                      inargs
1           strct strct strct                  strct                   strct                                                 strct
2                 slicw silcw  reversed        noinit                  targ     slicw slicw         elem  elem               targ
3                 lvsub lvsub  def             lvsub       lvsub lvsub amper    lvsub lvsub         iter  iter               amper
4     our   our   our   our    our       our   fake  state defer defer dbg      state state state   state state     state    dbg
5-6         dref        bool                   dref  dref  dref  dref  dref           bool          type  type               const(6)
7     intro intro intro intro  intro    implim intro intro intro intro intro    intro intro intro   intro intro     intro    nopar

If we use 6 for OUR_INTRO and shift 5-6 down to 4-5, that frees up 4
(aka OPpTARGET_MY) for use on split.

op.c:scalarvoid was testing the OPpOUR_INTRO flag on ops that don’t
take it, so it needed adjustment.
Fix Deparse OPpLVAL_INTRO handling in lists

The renumbering of private flags a few commits ago caused an exist-
ing Deparse bug to occur more often.  It was assuming that the
OPpLVAL_INTRO and OPpOUR_INTRO flags could occur on any ops for which
it did not have explicit exceptions.

This commit changes it to check for only those ops known to use those
flags, thus fixing bug #119815.
Allow OPpTARGET_MY optimisation for split

Many operators have a special SV allocated in the pad which is used
for return values (the target).  If we make that pad offset point to
a lexical variable, then we can optimise, say, $lexical = "$foo" into
just "$foo", where $lexical is stringify’s ‘target’.  And pp_stringify
doesn’t need to know any better.  We already do that for many ops.

This is safe to extend to split.  split only uses its target in this
code at the end:

    GETTARGET;
    PUSHi(iters);

so there is no danger of modifying its argument before reading it.
Optimise stringify away for "\Q$_" "\u$_" etc.
Make B constants depend on perl headers

Making the Makefile depend on the headers where the constants are
found will force Makefile.PL to be re-run, and it is Makefile.PL that
generates the constants.
Fix up f_map.t following "\L$_" optimisation