[MLIR][Affine][LICM] Mark users of `iter_args` variant

Prevent users of `iter_args` of an affine for loop from being hoisted
out of it. Otherwise, LICM leads to a violation of the SSA dominance
(as demonstrated in the added test case).

Fixes: https://bugs.llvm.org/show_bug.cgi?id=50103

Reviewed By: bondhugula, ayzhuang

Differential Revision: https://reviews.llvm.org/D102984