Reduce interpolation filter search

1. Do interpolation_filter_search after compound mode search.
This can skip interpolation_filter_search if compound_type_rd
is early terminated and should have more accurate interp_filters
decision.
2. Add early stop conditions in interpolation_filter_search.

Tested foreman, city, student CIF (30 frames, bitrate=500, Speed 1),
it is about 3.3% speed up and the average coding gain is about 0.25%.

STATS_CHANGED

Change-Id: Id65b7439d4edfeb7509f9e2e35079449d2f8c4f6
diff --git a/av1/encoder/block.h b/av1/encoder/block.h
index 003e59e..ed64e49 100644
--- a/av1/encoder/block.h
+++ b/av1/encoder/block.h
@@ -170,6 +170,7 @@
   InterpFilters filters;
   int_mv mv[2];
   int8_t ref_frames[2];
+  COMPOUND_TYPE comp_type;
 } INTERPOLATION_FILTER_STATS;
 
 typedef struct macroblock MACROBLOCK;