[mlir][Transforms] Add pass to perform sparse conditional constant propagation

This revision adds the initial pass for performing SCCP generically in MLIR. SCCP is an algorithm for propagating constants across control flow, and optimistically assumes all values to be constant unless proven otherwise. It currently supports branching control, with support for regions and inter-procedural propagation being added in followups.

Differential Revision: https://reviews.llvm.org/D78397