[InstCombine] Simplify boolean Phis with const inputs using CFG

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.

Differential Revision: https://reviews.llvm.org/D81375
Reviewed By: xbolva00