[mlir][scf] Implement structural conversion for 1:N type conversions.

This patch implements patterns for the newly introduced 1:N type
conversion utils for several ops of the SCF dialect. It also adds an
option to the existing test pass as well as test cases that applies the
patterns through the test pass.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D146959