Prepare to convert various types to size_t.

This makes some behaviorally-invariant changes to make certain code that
currently only works correctly with signed types work safely regardless of the
signedness of the types in question.  This is preparation for a future change
that will convert a variety of types to size_t.

There are also some formatting changes (e.g. converting "enum hack" usage to real consts) to make it simpler to just change "int" to "size_t" in the future to change the types of those constants.

BUG=none
R=andrew@webrtc.org, juberti@webrtc.org, kwiberg@webrtc.org
TBR=ajm

Review URL: https://codereview.webrtc.org/1174813003

Cr-Commit-Position: refs/heads/master@{#9413}
diff --git a/webrtc/modules/audio_coding/codecs/ilbc/encode.c b/webrtc/modules/audio_coding/codecs/ilbc/encode.c
index 1d46eff..3de8425 100644
--- a/webrtc/modules/audio_coding/codecs/ilbc/encode.c
+++ b/webrtc/modules/audio_coding/codecs/ilbc/encode.c
@@ -48,7 +48,7 @@
     IlbcEncoder *iLBCenc_inst /* (i/o) the general encoder
                                      state */
                           ){
-  int n, meml_gotten, Nfor, Nback;
+  int n, meml_gotten, Nfor;
   int16_t diff, start_pos;
   int index;
   int subcount, subframe;
@@ -379,15 +379,14 @@
 
   /* backward prediction of subframes */
 
-  Nback = iLBCbits_inst->startIdx-1;
-
-  if( Nback > 0 ){
+  if (iLBCbits_inst->startIdx > 1) {
 
     /* create reverse order vectors
        (The decresidual does not need to be copied since it is
        contained in the same vector as the residual)
     */
 
+    int Nback = iLBCbits_inst->startIdx - 1;
     WebRtcSpl_MemCpyReversedOrder(&reverseResidual[Nback*SUBL-1], residual, Nback*SUBL);
 
     /* setup memory */
@@ -425,11 +424,11 @@
       if (iLBCenc_inst->section == 1)
       {
         start_count = 0;
-        end_count = WEBRTC_SPL_MAX (2 - Nfor, 0);
+        end_count = (Nfor >= 2) ? 0 : (2 - NFor);
       }
       if (iLBCenc_inst->section == 2)
       {
-        start_count = WEBRTC_SPL_MAX (2 - Nfor, 0);
+        start_count = (Nfor >= 2) ? 0 : (2 - NFor);
         end_count = Nback;
       }
     }