compiler/nir: check whether var is an input in lower_fragcoord_wtrans
Otherwise the lowering pass might try to lower any other load from a deref if its data.location value happens to be zero. Fixes: 418c4c0d compiler/nir: extend lower_fragcoord_wtrans to support VARYING_SLOT_POS Signed-off-by:Gert Wollny <gert.wollny@collabora.com> Part-of: <mesa/mesa!10577> (cherry picked from commit e418710f)