[mlir][sparse] improved testing and codegen for semi-ring operations

The semi-ring blocks were simply "inlined" by the sparse compiler but
without any filtering or patching. This revision improves the analysis
(rejecting blocks that use non-invariant computations from outside
their blocks, except for linalg.index) and also improves the codegen
by properly patching up index computations (previous version crashed).

With a regression test. Also updated the documentation now that the
example code is properly working.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D128000