[Reducer] Speed up OperationPruner

[maerhart]

• Don't delete terminators because they are typically verified to be present and can usually not be removed without removing the whole block.
• Cache the user map instead of traversing the whole module again for each matched operation. This means that not all operations that could have been removed are actually removed in one iteration, but doing multiple iterations with this faster implementation is usually a lot faster than doing one more thorough iteration (there might be some corner cases where it's slightly slower, but probably not relevant in practice).

No unit test because it's only a performance optimization and the result should stay the same when performed until convergence.

Reduces the original file of #5307 with size 23672972 to size 2208 in 33 sec when only including the operation-pruner and cse patterns. Didn't measure time without this PR, because I didn't want to let it run for hours 😄

[fabianschuiki]

LGTM! Thanks for improving the reducer 🙏!