vtn: Fix atan2 for non-scalars.

vtn: Fix atan2 for non-scalars.

The if/then/else block was bogus, as it can only take a scalar
condition, and we need to select component-wise.  The GLSL IR
implementation of atan2 handles this by looping over components,
but I decided to try and do it vector-wise, and messed up.

For now, just bcsel.  It means that we do the atan1 math even if
all components hit the quick case, but it works, and presumably
at least one component will hit the expensive path anyway.