[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.
Depends On D93681