Several code cleanup changes in code to emit decoder tables:
- Start comments on each line at a fixed column for readibility.
- Combine repeated code to decode and emit ULEB128 into a single
function.
- Add helper `getDecoderOpName` to print decoder op.
- Print Filter/CheckField/predicate index values with those opcodes.

The instructions already have R6 register in the Uses list, there is no
need for an additional explicit `GPR:$skb` operand.

This simplifies intrinsic selection and makes the instructions decodable
without post-decoding pass inserting R6 operand.

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

## Purpose

This patch is one in a series of code-mods that annotate LLVM’s public
interface for export. This patch annotates the `llvm/Target` library.
These annotations currently have no meaningful impact on the LLVM build;
however, they are a prerequisite to support an LLVM Windows DLL (shared
library) build.

## Background

This effort is tracked in #109483. Additional context is provided in
[this
discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307),
and documentation for `LLVM_ABI` and related annotations is found in the
LLVM repo
[here](https://github.com/llvm/llvm-project/blob/main/llvm/docs/InterfaceExportAnnotations.rst).

A sub-set of these changes were generated automatically using the
[Interface Definition Scanner (IDS)](https://github.com/compnerd/ids)
tool, followed formatting with `git clang-format`.

The bulk of this change is manual additions of `LLVM_ABI` to
`LLVMInitializeX` functions defined in .cpp files under llvm/lib/Target.
Adding `LLVM_ABI` to the function implementation is required here
because they do not `#include "llvm/Support/TargetSelect.h"`, which
contains the declarations for this functions and was already updated
with `LLVM_ABI` in a previous patch. I considered patching these files
with `#include "llvm/Support/TargetSelect.h"` instead, but since
TargetSelect.h is a large file with a bunch of preprocessor x-macro
stuff in it I was concerned it would unnecessarily impact compile times.

In addition, a number of unit tests under llvm/unittests/Target required
additional dependencies to make them build correctly against the LLVM
DLL on Windows using MSVC.

## Validation

Local builds and tests to validate cross-platform compatibility. This
included llvm, clang, and lldb on the following configurations:

- Windows with MSVC
- Windows with Clang
- Linux with GCC
- Linux with Clang
- Darwin with Clang

In [1], a few new insns are proposed to expand BPF ISA to
  . fixing the limitation of existing insn (e.g., 16bit jmp offset)
  . adding new insns which may improve code quality
    (sign_ext_ld, sign_ext_mov, st)
  . feature complete (sdiv, smod)
  . better user experience (bswap)

This patch implemented insn encoding for
  . sign-extended load
  . sign-extended mov
  . sdiv/smod
  . bswap insns
  . unconditional jump with 32bit offset

The new bswap insns are generated under cpu=v4 for __builtin_bswap.
For cpu=v3 or earlier, for __builtin_bswap, be or le insns are generated
which is not intuitive for the user.

To support 32-bit branch offset, a 32-bit ja (JMPL) insn is implemented.
For conditional branch which is beyond 16-bit offset, llvm will do
some transformation 'cond_jmp' -> 'cond_jmp + jmpl' to simulate 32bit
conditional jmp. See BPFMIPeephole.cpp for details. The algorithm is
hueristic based. I have tested bpf selftest pyperf600 with unroll account
600 which can indeed generate 32-bit jump insn, e.g.,
        13:       06 00 00 00 9b cd 00 00 gotol +0xcd9b <LBB0_6619>

Eduard is working on to add 'st' insn to cpu=v4.

A list of llc flags:
  disable-ldsx, disable-movsx, disable-bswap,
  disable-sdiv-smod, disable-gotol
can be used to disable a particular insn for cpu v4.
For example, user can do:
  llc -march=bpf -mcpu=v4 -disable-movsx t.ll
to enable cpu v4 without movsx insns.

References:
  [1] https://lore.kernel.org/bpf/4bfe98be-5333-1c7e-2f6d-42486c8ec039@meta.com/

Differential Revision: https://reviews.llvm.org/D144829

SubtargetFeature.h is currently part of MC while it doesn't depend on
anything in MC. Since some LLVM components might have the need to work
with target features without necessarily needing MC, it might be
worthwhile to move SubtargetFeature.h to a different location. This will
reduce the dependencies of said components.

Note that I choose TargetParser as the destination because that's where
Triple lives and SubtargetFeatures feels related to that.

This issues came up during a JITLink review (D149522). JITLink would
like to avoid a dependency on MC while still needing to store target
features.

Reviewed By: MaskRay, arsenm

Differential Revision: https://reviews.llvm.org/D150549

The name `MCFixedLenDisassembler.h` is out of date after D120958.

Rename it as `MCDecoderOps.h` to reflect the change.

Reviewed By: myhsu

Differential Revision: https://reviews.llvm.org/D124987

All LLVM backends use MCDisassembler as a base class for their
instruction decoders. Use "const MCDisassembler *" for the decoder
instead of "const void *". Remove unnecessary static casts.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D122245

There's a few relevant forward declarations in there that may require downstream
adding explicit includes:

llvm/MC/MCContext.h no longer includes llvm/BinaryFormat/ELF.h, llvm/MC/MCSubtargetInfo.h, llvm/MC/MCTargetOptions.h
llvm/MC/MCObjectStreamer.h no longer include llvm/MC/MCAssembler.h
llvm/MC/MCAssembler.h no longer includes llvm/MC/MCFixup.h, llvm/MC/MCFragment.h

Counting preprocessed lines required to rebuild llvm-project on my setup:
before: 1052436830
after:  1049293745

Which is significant and backs up the change in addition to the usual benefits of
decreasing coupling between headers and compilation units.

Discourse thread: https://discourse.llvm.org/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D119244