audio_coding/codec/ilbc: Removed usage of macro WEBRTC_SPL_MUL_16_16

The macro is in C defined as
#define WEBRTC_SPL_MUL_16_16(a, b) ((int32_t) (((int16_t)(a)) * ((int16_t)(b))))
(For definition on ARMv7 and MIPS, see common_audio/signal_processing/include/spl_inl_armv7.h and common_audio/signal_processing/include/spl_inl_mips.h)

The replacement consists of
- avoiding casts to int16_t if inputs already are int16_t
- adding explicit cast to <type> if result is assigned to <type> (other than int or int32_t)

Some other minor code cleanup also exists.

BUG=3348, 3353
TESTED=locally on Mac and trybots
R=henrik.lundin@webrtc.org, kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/34179004

Cr-Commit-Position: refs/heads/master@{#8358}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8358 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/codecs/ilbc/encode.c

@@ -154,7 +154,7 @@ void WebRtcIlbcfix_EncodeImpl(
 
     index = (iLBCbits_inst->startIdx-1)*SUBL;
     max=WebRtcSpl_MaxAbsValueW16(&residual[index], 2*SUBL);
-    scale=WebRtcSpl_GetSizeInBits(WEBRTC_SPL_MUL_16_16(max,max));
+    scale = WebRtcSpl_GetSizeInBits((uint32_t)(max * max));
 
     /* Scale to maximum 25 bits so that the MAC won't cause overflow */
     scale = scale - 25;