Refactor of the llvm-tblgen source into:
- a "Basic" library, which contains the bare minimum utilities to build
`llvm-min-tablegen`
- a "Common" library which contains all of the helpers for TableGen
backends. Such helpers can be shared by more than one backend, and even
unit tested (e.g. CodeExpander is, maybe we can add more over time)

Fixes #80647

DecoderEmitter and CodeEmitterGen perform repeated linear walks over the
entire instruction list. This patch eliminates two more such walks.

The eliminated traversals visit every instruction merely to determine
whether the target has variable length encodings. For a target with
variable length encodings, the original any_of will terminate quickly.
But all targets other than M68k use fixed length encodings and thus
any_of must visit the entire instruction list.

These are unnecessary since C++17.

```
find llvm/utils/TableGen -iname "*.h" -o -iname "*.cpp" | xargs clang-format-16 -i
```

Split from #80847

This patch removes CodeGenTarget::InstrToIntMap, using instead a new
member CodeGenInstruction::EnumVal to store each enum value. This value
is computed and set by CodeGenTarget::computeInstrsByEnum and queried by
CodeGenTarget::getInstrIntValue.

Currently, for some tables involving a single primary key field which is
integral and densely numbered, a direct lookup is generated rather than
a binary search. This patch extends the direct lookup function
generation to instructions, where the integral value corresponds to the
instruction's enum value.

While this isn't as common as for other tables, it does occur in at
least one downstream backend and one in-tree backend.

Added a unit test and minimally updated the documentation.

`Fusion` is inherited from `SubtargetFeature` now. Each definition
of `Fusion` will define a `SubtargetFeature` accordingly.

Method `getMacroFusions` is added to `TargetSubtargetInfo`, which
returns a list of `MacroFusionPredTy` that will be evaluated by
MacroFusionMution.

`getMacroFusions` will be auto-generated if the target has `Fusion`
definitions.

See https://discourse.llvm.org/t/rfc-overhauled-mir-patterns-for-globalisel-combiners/72264

This is a complete overrhaul of the recently-added GlobalISel Match Table backend which adds
support for MIR patterns for both match and apply patterns.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D156315