Moved dequant idct into common

These functions are now used by the encoder.
This is WIP with the goal of creating a common idct/add for
the encoder and decoder.  A boost of 1.8% was seen for
the HD rt test clip used.

[Tero] Added needed changes to ARM side.

Change-Id: Ibbb8000be09034203d7adffc457d3c3f8b06a5bf

vp8/encoder/rdopt.c

@@ -2166,7 +2166,7 @@ void vp8_rd_pick_inter_mode(VP8_COMP *cpi, MACROBLOCK *x, int recon_yoffset,
                 continue;
 
             vp8_set_mbmode_and_mvs(x, this_mode, &mode_mv[this_mode]);
-            vp8_build_inter16x16_predictors_mby(&x->e_mbd);
+            vp8_build_inter16x16_predictors_mby(&x->e_mbd, x->e_mbd.predictor, 16);
 
             if (cpi->active_map_enabled && x->active_ptr[0] == 0) {
                 x->skip = 1;