[mlir][memref] Unranked support for extract_aligned_pointer_as_index (#93908)

memref.extract_aligned_pointer_as_index currently does not support
unranked inputs. This lack of support interferes with the folding
operations in the expand-strided-metadata pass.

    %r = memref.reinterpret_cast %arg0 to
        offset: [0],
        sizes: [],
        strides: [] : memref<*xf32> to memref<f32>

%i = memref.extract_aligned_pointer_as_index %r : memref<f32> -> index

Patterns like this occur when bufferizing operations on unranked
tensors.

This change modifies the extract_aligned_pointer_as_index operation to
support unranked inputs with corresponding support in the MemRef->LLVM
conversion.

Co-authored-by: Spenser Bauman <sabauma@fastmail>