This improves type safety and is less verbose. Use SimpleTy only where
an integer is needed like switches or emitting a VBR.

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

Identified with modernize-use-using.

- Move standalone functions and variables out of anonymous namespace and
make them static.
- Eliminate `namespace llvm {}` wrapping all code in .cpp files, and
instead use namespace qualifier to define such functions
(https://llvm.org/docs/CodingStandards.html#use-namespace-qualifiers-to-implement-previously-declared-functions)
- Add namespace for X86DisassemblerShared.h.

cPTR is a wildcard CHERI capability value type, used analogously to iPTR. This allows TableGen patterns to abstract over CHERI capability widths.

Co-authored-by: Jessica Clarke <jrtc27@jrtc27.com>

This is a generalization of the LookupPtrRegClass mechanism.
AMDGPU has several use cases for swapping the register class of
instruction operands based on the subtarget, but none of them
really fit into the box of being pointer-like.

The current system requires manual management of an arbitrary integer
ID. For the AMDGPU use case, this would end up being around 40 new
entries to manage.

This just introduces the base infrastructure. I have ports of all
the target specific usage of PointerLikeRegClass ready.

`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`.

Rename `getXYZInstructionsByEnumValue()` to just `getXYZInstructions`
and drop the `ByEnumValue` in the name.

Use llvm::fill instead of std::fill