[mlir] Minor fixes after removing types from attributes

D130092 removed types from attributes. This patch fixes a minor
issues what was exposed when integrating that change in IREE. An
explicit cast is needed so that the template type of `makeArrayRef`
is automatically deduced.

Co-authored-by: Lei Zhang <antiagainst@google.com>

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D131604