[mlir] Fix call op conversion in bare-ptr calling convention

We hit an llvm_unreachable related to unranked memrefs for call ops
with scalar types. Removing the llvm_unreachable since the conversion
should gracefully bail out in the presence of unranked memrefs. Adding
tests to verify that.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D88709