[MLIR][Liveness] Add `currentlyLiveValues`, a way to get a set of values that...

[MLIR][Liveness] Add `currentlyLiveValues`, a way to get a set of values that are live as of a given operation.

This change allows the user of LivenessBlockInfo to specify an op within the block and get a set of all values that are live as of that op. Semantically it relies on having a dominance-based region that has ordered operations. For DFG regions, computing liveness statically this way doesn't really make sense, it likely needs to be done at runtime.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D129447