[MLIR] Make promote single iteration optional on affine-loop-normalize

Make promote single iteration optional on affine-loop-normalize:
introduce a command-line flag and an argument on the utility. Disable it
by default since such a promotion isn't normally expected with loop
normalization: it could drop certain structure or information on the
loops that a user wanted to preserve.

Reviewed By: dcaballe

Differential Revision: https://reviews.llvm.org/D137605