AMDGPU: Move permlane discard vdst_in optimization

This case can be handled as a regular selection pattern, so move it
out of the weird post-isel folding code which doesn't have an exactly
equivalent place in GlobalISel.

I think it doesn't make much sense to do this optimization here
though, and it would be more useful in instcombine. There's not really
any new information that will be gained during lowering since these
inputs were known from the beginning.