For the pattern of IR (%if terminates with a divergent branch.),
divergence analysis will report %phi as uniform to help optimal code
%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.