This patch adds simplification for pattern:

if (cond) / \ ... ... \ / p = phi [true] [false] ... br p, succ_1, succ_2

If we can prove that top block's branches dominate respective

inputs of a block that has a Phi with constant inputs, we can

use the branch condition (maybe inverted) instead of Phi.

This will make proofs of implication for further jump threading

more transparent.