This PR adds scalar type to convert_layout op's result and operand. It
also enhance convert_layout pattern in wg-to-sg, unrolling, and
sg-to-lane distribution.
It is to support reduction to scalar, whether currently the layout
propagation doesn't support scalar to carry any layout. The design
choice to insert convert_layout op after reduction-to-scalar op to
record the layout information permanently across the passes.
28e2fa3247