- 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

Increase width of ImplicitOffset to `int` from `short` to allow more
table capacity.
Reorder the elements to maintain natural alignment.

- Use range for loops for processor models and schedule classes.
- Cleanup duplicated or unused iterators in CodeGenSchedule.h

- Change InstrInfoEmitter to emit OpName as an enum class
  instead of an anonymous enum in the OpName namespace.
- This will help clearly distinguish between values that are 
  OpNames vs just operand indices and should help avoid
  bugs due to confusion between the two.
- Rename OpName::OPERAND_LAST to NUM_OPERAND_NAMES.
- Emit declaration of getOperandIdx() along with the OpName
  enum so it doesn't have to be repeated in various headers.
- Also updated AMDGPU, RISCV, and WebAssembly backends
  to conform to the new definition of OpName (mostly
  mechanical changes).

- Use range for loops and `enumerate` in a few places.
- Use `StringRef` for `TargetName` in `InstrInfoEmitter::run`.
- Use `\n` character for new line instead of string.
- Use StringRef in `InstrNames` (instead of std::string) and 
  avoid string copies.

Delete `getLogicalOperandType` function from InstrInfoEmitter as no
backend seems to use it.

- Detect whether logical operand mapping/named operand mappings have 
  been enabled in a previous pass over instructions and execute the
  relevant emission code only if those mappings are enabled.
- For these mappings, skip the fixed set of predefined instructions as
  they won't have these mappings enabled.
- Emit operand type mappings only for X86 target, as they are only used
  by X86 and look for X86 specific `X86MemOperand`.
- Cleanup `emitOperandTypeMappings` code: remove code to handle empty
  instruction list and use range for loops.

- Use `Emitter::OptClass` to invoke `InstrInfoEmitter::run` and
eliminate the `EmitInstrInfo` function.

- Looks like this sentinel value is used in some downstream backends, so
restore emitting it.
- It now also has the correct value (earlier code may have emitted an
incorrect value for OPERAND_LAST and hence it was removed in
https://github.com/llvm/llvm-project/pull/124960)