st/glsl_to_tgsi: fix ir_assignment hack doing bad things for doubles

This hack for fixing gl_FragDepth apparantly caused a GLSL shader
outputting a single double to try and output a dvec4, but we hadn't
assigned outputs for the secondary bit.

This avoids going into the hack code for scalar doubles.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/mesa/state_tracker/st_glsl_to_tgsi.cpp

@@ -2611,6 +2611,7 @@ glsl_to_tgsi_visitor::visit(ir_assignment *ir)
       assert(!ir->lhs->type->is_scalar() && !ir->lhs->type->is_vector());
       l.writemask = WRITEMASK_XYZW;
    } else if (ir->lhs->type->is_scalar() &&
+              !ir->lhs->type->is_double() &&
               ir->lhs->variable_referenced()->data.mode == ir_var_shader_out) {
       /* FINISHME: This hack makes writing to gl_FragDepth, which lives in the
        * FINISHME: W component of fragment shader output zero, work correctly.