[InstCombine] Combine select & Phi by same condition

This patch transforms
```
p = phi [x, y]
s = select cond, z, p
```
with
```
s = phi[x, z]
```
if we can prove that the Phi node takes values basing on select's condition.

Differential Revision: https://reviews.llvm.org/D82072
Reviewed By: nikic