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