avcodec/mpv_reconstruct_mb_template: Optimize always-true branch away

There are only two mpegvideo decoders that use another
(software) pixel format than YUV420: MPEG-1/2 and
the MPEG-4 studio profile. Neither of these use this part
of the code, so one can optimize the 422 code away when
this code is compiled for the decoder.

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

libavcodec/mpv_reconstruct_mb_template.c

@@ -172,7 +172,8 @@ void mpv_reconstruct_mb_internal(MpegEncContext *s, int16_t block[12][64],
                 add_dequant_dct(s, block[3], 3, dest_y + dct_offset + block_size, dct_linesize, s->qscale);
 
                 if (!CONFIG_GRAY || !(s->avctx->flags & AV_CODEC_FLAG_GRAY)) {
-                    if (s->chroma_y_shift) {
+                    av_assert2(IS_ENCODER || s->chroma_y_shift);
+                    if (!IS_ENCODER || s->chroma_y_shift) {
                         add_dequant_dct(s, block[4], 4, dest_cb, uvlinesize, s->chroma_qscale);
                         add_dequant_dct(s, block[5], 5, dest_cr, uvlinesize, s->chroma_qscale);
                     } else {