nir: Only do gl_FrontFacing workaround in glsl_to_nir for the FS.

nir: Only do gl_FrontFacing workaround in glsl_to_nir for the FS.

Vertex shaders can have shader inputs where location happens to be
VARYING_SLOT_FACE.  Without predicating this on the shader stage,
we suddenly end up with load_front_face intrinsics in vertex shaders,
which is nonsensical.

Fixes spec/arb_vertex_buffer_object/pos-array when using NIR for VS.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>