[mlir][Affine][Vector] Add initial support for 'iter_args' to Affine vectorizer.

This patch adds support for vectorizing loops with 'iter_args' when those loops
are not a vector dimension. This allows vectorizing outer loops with an inner
'iter_args' loop (e.g., reductions). Vectorizing scenarios where 'iter_args'
loops are vector dimensions would require more work (e.g., analysis,
generating horizontal reduction, etc.) not included in this patch.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D97892