ans: Optimize rabs unbiased bit decode

Change-Id: Icb4ab1de851db9f4016af8757e17fcb935a2db5a

aom_dsp/ansreader.h

diff --git a/aom_dsp/ansreader.h b/aom_dsp/ansreader.h
index faddf42..fcd9ca9 100644
--- a/aom_dsp/ansreader.h
+++ b/aom_dsp/ansreader.h
@@ -84,9 +84,16 @@
 // Decode one rABS encoded boolean where the probability of the value being zero
 // is one half.
 static INLINE int rabs_read_bit(struct AnsDecoder *ans) {
-  // TODO(aconverse@google.com): Provide an optimized implementation of this
-  // routine.
-  return rabs_read(ans, 128);
+#if ANS_MAX_SYMBOLS
+  if (ans->symbols_left-- == 0) {
+    ans_read_reinit(ans);
+    ans->symbols_left--;
+  }
+#endif
+  unsigned state = refill_state(ans, ans->state);
+  const int value = !!(state & 0x80);
+  ans->state = ((state >> 1) & ~0x7F) | (state & 0x7F);
+  return value;
 }
 
 struct rans_dec_sym {