[mlir][LLVM] Fix crash when using opaque pointers in function signatures

Using opaque pointers in function signatures leads to an attempt to recursively convert all types, including sub types in LLVM types. In the case of LLVM pointers, it may not have a subtype aka element type if it is opaque which would then lead to a null pointer dereference.

Differential Revision: https://reviews.llvm.org/D124291