Saga of m68k PIC continues

[Jim Wilson]

UNSPECs aren't normal operands.  They are unspecified operands, and hence are
not handled by any machine independent code.  They must be handled by machine
dependent code.  Usually, you add explicit patterns containing UNSPEC to the
md file to get them recognized.  This allows you to emit code for unusual
operations that can't be directly represented in RTL.  If you can represent
what you want in normal RTL, then you should not be using UNSPECs.

You might be able to write predicates that accept UNSPEC operands.  It isn't
the usual way of using them though.

Don't forget about PREDICATE_CODES.  If you are adding new predicates, or
modifying the operands accepted by a predicate, you need to update this
macro too.

Jim