[mlir][MemRef] Fix the lowering of extract_strided_metadata

The first result of the extract_strided_metadata operation is a MemRef,
not a naked pointer.
This patch fixes the lowering of this operation in MemRefToLLVM so that
we properly materialize the full MemRef structure and not just the base,
naked, pointer.

Differential Revision: https://reviews.llvm.org/D137364