[mlir] Extended Dominance analysis with a function to find the nearest common...

[mlir] Extended Dominance analysis with a function to find the nearest common dominator of two given blocks.

The Dominance analysis currently misses a utility function to find the nearest common dominator of two given blocks. This is required for a huge variety of different control-flow analyses and transformations. This commit adds this function and moves the getNode function from DominanceInfo to DominanceInfoBase, as it also works for post dominators.

Differential Revision: https://reviews.llvm.org/D75507