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>

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.

The added tests used to crash when attempting to dereference a nullptr
MIOpInfo or call MIOpInfo->getArg(0) on an empty MIOpInfo dag.

I believe it became no-op with the removal of the "positionally encoded
operands" functionality (b87dc356 is the last commit in the series).

There are no changes in the generated files.

Also change its name to start with a lowercase letter and update
the doxygen comment to conform to the coding standard.

- Change `hadOperandNamed` to return index as std::optional and rename
it to `findOperandNamed`.
- Change `SubOperandAlias` to return std::optional and rename it to
`findSubOperandAlias`.

- Change `Name`, `SubopNames`, `PrinterMethodName`, and
`EncoderMethodNames` to be stored as StringRef.
- Also changed `CheckComplexPatMatcher::Name` to StringRef as a fallout
from the above.

Verified that all the tablegen generated files within LLVM are
unchanged.

- Also eliminate unneeded std::string() around some literal strings.