[mlir] Give each OpResult its own use list

This revision removes the multi use-list to ensure that each result gets its own. This decision was made by doing some extensive benchmarking of programs that actually use multiple results. This results in a size increase of 1-word per result >1, but the common case of 1-result remains unaffected. A side benefit is that 0-result operations now shrink by 1-word.

Differential Revision: https://reviews.llvm.org/D78701