Combine Huffman cost and bit entropy into one loop

The same computation was done for both values: go over two buffers,
sum them up, and take a decision on the sum at each iteration.

MIPS32 code has been disabled for now, pending a code update.

Change-Id: I997984326f7092b3dbb8cfa1e524bd8132b2ab9d

src/dsp/lossless_enc_mips32.c

@@ -400,7 +400,13 @@ WEBP_TSAN_IGNORE_FUNCTION void VP8LEncDspInitMIPS32(void) {
   VP8LExtraCost = ExtraCost;
   VP8LExtraCostCombined = ExtraCostCombined;
   VP8LHuffmanCostCount = HuffmanCostCount;
+// TODO(mips team): rewrite VP8LGetCombinedEntropy (which used to use
+// HuffmanCostCombinedCount) with MIPS optimizations
+#if 0
   VP8LHuffmanCostCombinedCount = HuffmanCostCombinedCount;
+#else
+ (void)HuffmanCostCombinedCount;
+#endif
   VP8LHistogramAdd = HistogramAdd;
 }