AMDGPU: Add a pass to rewrite certain undef in PHI

For the pattern of IR (%if terminates with a divergent branch.),
divergence analysis will report %phi as uniform to help optimal code
generation.
```
  %if
  | \
  | %then
  | /
  %endif: %phi = phi [ %uniform, %if ], [ %undef, %then ]
```
In the backend, %phi and %uniform will be assigned a scalar register.
But the %undef from %then will make the scalar register dead in %then.
This will likely cause the register being over-written in %then. To fix
the issue, we will rewrite %undef as %uniform. For details, please refer
the comment in AMDGPURewriteUndefForPHI.cpp. Currently there is no test
changes shown, but this is mandatory for later changes.

Reviewed by: sameerds

Differential Revision: https://reviews.llvm.org/D133840