[mlir][sparse][gpu][nvidia] add pruning step and check to 2:4 matrix multiplication

(1) without the check, the results may silently be wrong, so check is needed
(2) add pruning step to guarantee 2:4 property

Note, in the longer run, we may want to split out the pruning step somehow,
or make it optional.

Reviewed By: K-Wu

Differential Revision: https://reviews.llvm.org/D155320