freedreno/ir3: assign varying locations later

freedreno/ir3: assign varying locations later

Rather than assigning inloc up front, when we don't yet know if it will
be unused, assign it last thing before the legalize pass.

Also, realize when inputs are unused (since for frag shader's we can't
rely on them being removed from ir->inputs[]).  This doesn't make sense
if we don't also dynamically assign the inloc's, since we could end up
telling the hw the wrong # of varyings (since we currently assume that
the # of varyings and max-inloc are related..)

Signed-off-by: Rob Clark <robclark@freedesktop.org>