LLVM IR Conversion: keep LLVM dialect types as is during conversion

When converting to the LLVM IR Dialect, it is possible for the input IR to
contain LLVM IR Dialect operation and/or types, for example, some functions may
have been coverted to the LLVM IR Dialect already, or may have been created
using this dialect directly.  Make sure that type conversion keeps LLVM IR
Dialect types unmodified and does not error out.  Operations are already kept
as is.

PiperOrigin-RevId: 240574972