[mlir][sparse] implement full reduction "scalarization" across loop nests

The earlier reduction "scalarization" was only applied to a chain of
*innermost* and *for* loops. This revision generalizes this to any
nesting of for- and while-loops. This implies that reductions can be
implemented with a lot less load and store operations. The chaining
is implemented with a forest of yield statements (but not as bad as
when we would also include the while-induction).

Fixes https://bugs.llvm.org/show_bug.cgi?id=52311

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D113078