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

---------

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

/llvm-project/llvm/utils/TableGen/Common/CodeGenTarget.cpp:286:12:
 error: variable 'SkippedInsts' set but not used [-Werror,-Wunused-but-set-variable]
  unsigned SkippedInsts = 0;
           ^
1 error generated.

Allow a target to steal the definition of a generic pseudoinstruction
and remap the operands. This works by defining a new instruction, which
will simply swap out the emitted entry in the InstrInfo table.

This is intended to eliminate the C++ half of the implementation
of PointerLikeRegClass. With RegClassByHwMode, the remaining usecase
for PointerLikeRegClass are the common codegen pseudoinstructions.
Every target maintains its own copy of the generic pseudo operand
definitions anyway, so we can stub out the register operands with
an appropriate class instead of waiting for runtime resolution.

In the future we could probably take this a bit further. For example,
there is a similar problem for ADJCALLSTACKUP/DOWN since they depend
on target register definitions for the stack pointer register.

Per https://llvm.org/docs/CodingStandards.html#include-as-little-as-possible
this improves compilation time, while not being too intrusive on the
codebase.

Identified with modernize-use-equals-default.

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.

`Target.getInstructions()` is called by virtually all TableGen backends.
It is slow, and one of the two factors is the use of an expensive
predicate in `llvm::sort`. This change speeds up sorting by 10x.

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

- Add various instruction subclass/sub-slice accessors to
`CodeGenTarget`.
- Delete unused `inst_begin` and `inst_end` iterators.
- Rename `Instructions` to `InstructionMap` and `getInstructions` to
`getInstructionMap` to better represent their meaning.
- Use these new accessors in InstrInfoEmitter

This method doesn't use anything from CodeGenTarget, so it seems to
belong in CodeGenRegBank.