There are cases in SPIR-V shaders where values need to be yielded from
the selection region to make valid MLIR. For example (part of the SPIR-V
shader decompiled to GLSL):
```
bool _115
if (_107)
{
// ...
float _200 = fma(...);
// ...
_115 = _200 < _174;
}
else
{
_115 = _107;
}
bool _123;
if (_115)
{
// ...
float _213 = fma(...);
// ...
_123 = _213 < _174;
}
else
{
_123 = _115;
}
````
This patch extends `mlir.selection` so it can return values.
`mlir.merge` is used as a "yield" operation. This allows to maintain a
compatibility with code that does not yield any values, as well as, to
maintain an assumption that `mlir.merge` is the only operation in the
merge block of the selection region.
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