[mlir][sparse] mask reduction update

Reduction updates should be masked, just like the load and stores.
Note that alternatively, we could use the fact that masked values are
zero of += updates and mask invariants to get this working but that
would not work for *= updates. Masking the update itself is cleanest.
This change also replaces the constant mask with a broadcast of "true"
since this constant folds much better for various folding patterns.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D98000