Remove C++11 calls from intelligibility_utils

The C++11 here was overkill. This replaces it with simpler logic that
covers all cases encountered so far in practice.

The problem was previously brought up here: https://codereview.webrtc.org/1250663007/

BUG=427718, 487341, webrtc:4866
R=andrew@webrtc.org

Review URL: https://codereview.webrtc.org/1242943008

Cr-Original-Commit-Position: refs/heads/master@{#9627}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3ab2f14d56db91665002f36f899c0a97f926d91f
3 files changed