Refactor audio_coding/isac: removed usage of macro WEBRTC_SPL_LSHIFT_W32

The macro is defined as
#define WEBRTC_SPL_LSHIFT_W32(a, b) ((a) << (b))
It is a trivial operation that need no macro. In fact it may be confusing for to the user, since it can be interpreted as having an implicit cast to int32_t.

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

Review URL: https://webrtc-codereview.appspot.com/44659004

Cr-Original-Commit-Position: refs/heads/master@{#8801}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6069032ebb9026bc22ae18d1c4b8e8315140a907