[mlir] LLVM lowering: don't use calling convention in op returns

Conversions to the LLVM dialect have an option to use the "bare pointer"
calling convention that converts memref types differently than the
default convention. It has crept into the conversion of operations that
are not related to calls but do require multiresult-to-struct packing.
Use a similar mechanism for the latter without using the calling
convention.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D148086