GL_EXT_subgroupuniform_qualifier and flow control rejoining

cc @sheredom

Consider this example:

```
subgroupuniformEXT int x;
if ((gl_InvocationID & 1) != 0) {
x = 0
} else {
x = 1
}
use x;
```

Both assignments can be correctly decorated as Uniform in the SPIR-V, but when the if and else blocks rejoin there would be an OpPhi in the SPIR-V that is not Uniform. It seems like the GLSL extension spec doesn't clarify what happens in this case. Is it an error? Undefined behavior? Does the frontend compiler stop propagating Uniform when flow control rejoins?

Begin by reading the GL_EXT_subgroupuniform_qualifier extension specification and the SPIR-V rules for the shown if/else rejoin and OpPhi. Compare the example's Uniform decorations with the specification to determine whether the behavior is an error, undefined, or requires a defined propagation rule; done means the specification clearly answers this case.

Written by the indexing model from the issue text.