nir: add lowering stage for user-clip-planes / clipdist

The vertex shader lowering adds calculation for CLIPDIST, if needed
(ie. user-clip-planes), and the frag shader lowering adds conditional
kills based on CLIPDIST value (which should be treated as a normal
interpolated varying by the driver).

Note that this won't quite do the right thing in the face of MSAA plus
user-clip-planes, since all the samples would be killed or not (rather
than potentially only a portion of them).  But it's better than no UCP
support at all for drivers that don't have this in hw.

Signed-off-by: Rob Clark <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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