[mlir] Remove LLVMType, LLVM dialect types now derive Type directly

BEGIN_PUBLIC
[mlir] Remove LLVMType, LLVM dialect types now derive Type directly

This class has become a simple `isa` hook with no proper functionality.
Removing will allow us to eventually make the LLVM dialect type infrastructure
open, i.e., support non-LLVM types inside container types, which itself will
make the type conversion more progressive.

Introduce a call `LLVM::isCompatibleType` to be used instead of
`isa<LLVMType>`. For now, this is strictly equivalent.
END_PUBLIC

Depends On D93681

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D93713