`Predicates` and `Features` fields serve the same purpose. They should
be kept in sync, but not all predicates are based on features. This
resulted in introducing dummy features for that only reason.

This patch removes `Features` field and changes TableGen emitters to use
`Predicates` instead.

Historically, predicates were written with the assumption that the
checking code will be used in `SelectionDAGISel` subclasses, meaning
they will have access to the subclass variables, such as `Subtarget`.
There are no such variables in the generated
`GenSubtargetInfo::getHwModeSet()`, so we need to provide them. This can
be achieved by subclassing `HwModePredicateProlog`, see an example in
`Hexagon.td`.

This will make it possible for tablegen to make subtarget
dependent decisions without adding new arguments to every
target.

---------

Co-authored-by: Sergei Barannikov <barannikov88@gmail.com>

I believe it became no-op with the removal of the "positionally encoded
operands" functionality (b87dc356 is the last commit in the series).

There are no changes in the generated files.

Turn a funnel shift by N in the range `121..128` into a funnel shift in
the opposite direction by `128 - N`. Because there are dedicated
instructions for funnel shifts by values smaller than 8, this emits
fewer instructions.

This additional rule is useful because LLVM appears to canonicalize
`fshr` into `fshl`, meaning that the rules for `fshr` on values less
than 8 would not match on organic input.

Simplify min/max instruction matching by making the related
SelectionDAG operations legal.

Add patterns to match (signed and unsigned) saturated
truncation based on open-coded min/max patterns.

Fixes https://github.com/llvm/llvm-project/issues/153655

Extract fixed functions generated by decoder emitter into a new
MCDecoder.h header.

That is, on all targets except ARM and AArch64.
This field used to be required due to a bug, it was fixed long ago
by 23423c0ea8d414e56081cb6a13bd8b2cc91513a9.

The calling convention lowering now has access to OrigTy, so use that to
detect short vectors.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

This ensures that the required fields are set, and also makes the
construction more convenient.