Whether lowres is in use or not is inlined in
mpv_reconstruct_mb_internal(), so one can use the fact
that lowres is always zero during encoding to evaluate
the checks for whether one is encoding or not at compile-time
when one is in lowres mode.
Also reorder the main check to check for whether it is an encoder
first to shortcircuit it in the common case of a decoder.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>