Refactor common_audio/signal_processing: Removed usage of WEBRTC_SPL_MUL_16_16_RSFT

The macro is defined as
#define WEBRTC_SPL_MUL_16_16_RSFT(a, b, c) \
(WEBRTC_SPL_MUL_16_16(a, b) >> (c))

where the latter macro is in C defined as
#define WEBRTC_SPL_MUL_16_16(a, b) \
((int32_t) (((int16_t)(a)) * ((int16_t)(b))))
(For definitions on ARMv7 and MIPS, see common_audio/signal_processing/include/spl_inl_{armv7,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)
- minor cleanups like remove of unnecessary parentheses and style changes

BUG=3348, 3353
TESTED=locally on Linux for both fixed and floating point and trybots
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49499004

Cr-Commit-Position: refs/heads/master@{#8853}

webrtc/common_audio/signal_processing/ilbc_specific_functions.c

@@ -32,8 +32,7 @@ void WebRtcSpl_ReverseOrderMultArrayElements(int16_t *out, const int16_t *in,
     const int16_t *winptr = win;
     for (i = 0; i < vector_length; i++)
     {
-        (*outptr++) = (int16_t)WEBRTC_SPL_MUL_16_16_RSFT(*inptr++,
-                                                               *winptr--, right_shifts);
+      *outptr++ = (int16_t)((*inptr++ * *winptr--) >> right_shifts);
     }
 }
 
@@ -47,8 +46,7 @@ void WebRtcSpl_ElementwiseVectorMult(int16_t *out, const int16_t *in,
     const int16_t *winptr = win;
     for (i = 0; i < vector_length; i++)
     {
-        (*outptr++) = (int16_t)WEBRTC_SPL_MUL_16_16_RSFT(*inptr++,
-                                                               *winptr++, right_shifts);
+      *outptr++ = (int16_t)((*inptr++ * *winptr++) >> right_shifts);
     }
 }