nir: Fix fddy swizzles in nir_lower_wpos_ytransform().

The original value might have been swizzled.  That's taken care of in
the fmul source - we don't want to reswizzle it again.

Fixes validation failures in glsl-derivs-varyings on a branch of mine
which uses this pass in i965.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>
1 file changed