Make B::OP::slabbed, folded, etc. actually work

B::OP::slabbed       = 47
B::OP::savefree      = 48
B::OP::static        = 49
B::OP::folded        = 50

Added by 3164fde474, these have never worked, because the alias index
check in B::OP::next was never updated.

Instead of hard-coding the number, use C_ARRAY_LENGTH(...).
Remove OPpCONST_FOLDED

Now that we have op->op_folded, we don’t need OPpCONST_FOLDED any
more.  In removing it, I modified B::Concise to output op_folded the
way OPpCONST_FOLDED was output before, since it can be helpful to have
it when reading op dumps.
Alphabetise perldiag
Fix minor flagging bug in op.c:fold_constants

Nothing is making use of this in at present in the affected cases.
What this code does is mark an op as being folded so that 1+2 can
be distinguished from 3.  qq"foo" was erroneously being marked as
folded, unlike "foo".  Constant folding is part of the way that
quote-like operators are implemented.  Conceptually there is no
folding here, so we should pretend it did not happen.  This was a
partial cause of bug #116086.
Tweak B::Concise tests following OPpCONST_FOLDED removal