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
I originally reverted the patch because it was causing performance
issues, but now I think it's just enabling simplify-cfg to do
something that I don't want instead :)
Sorry for the noise.
This reverts commit 3e39760f8e.
We can simplify
```
icmp <pred> phi(C1, C2, ...), C
```
with
```
phi(icmp(C1, C), icmp(C2, C), ...)
```
provided that all comparison of constants are constants themselves.
Differential Revision: https://reviews.llvm.org/D81151
Reviewed By: lebedev.ri
Max Kazantsev