[TableGen] Add useDeprecatedPositionallyEncodedOperands option.

Summary:
The existing undefined-bitfield-to-operand matching behavior is very
hard to understand, due to the combination of positional and named
matching. This can make it difficult to track down a bug in a target's
instruction definitions.

Over the last decade, folks have tried to work-around this in various
ways, but it's time to finally ditch the positional matching. With
https://reviews.llvm.org/D131003, there are no longer cases that
_require_ positional matching, and it's time to start removing usage
and support for it.

Therefore: add a (default-false) option, and set it to true only in
those targets that require positional matching today. Subsequent
changes will start cleaning up additional in-tree targets.

NOTE TO OUT OF TREE TARGET MAINTAINERS:

If this change breaks your build, you may restore the previous
behavior simply by adding:
  let useDeprecatedPositionallyEncodedOperands = 1;
to your target's InstrInfo tablegen definition. However, this is
temporary -- the option will be removed in the future.

If your target does not set 'decodePositionallyEncodedOperands', you
may thus start migrating to named operands. However, if you _do_
currently set that option, I recommend waiting until a subsequent
change lands, which adds decoder support for named sub-operands.

Differential Revision: https://reviews.llvm.org/D134073