nir: Make nir_lower_wpos_ytransform() match FragCoord by location.

nir: Make nir_lower_wpos_ytransform() match FragCoord by location.

gl_FragCoord is a shader input with location == VARYING_SLOT_POS.
ARB_fragment_programs have an equivalent input at VARYING_SLOT_POS,
but it isn't called gl_FragCoord.  We do want to transform it.

Matching by location guarantees we catch both.

Fixes several fp tests on a branch which uses this pass on i965.

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