ARM: speed up bitreader by avoiding tables
ARM: speed up bitreader by avoiding tables

(and using BitsLog2Floor() from utils.h instead)

9-10% speed-up, apparently

Change-Id: I9acae4a4dceb1ddcc99306f99b722079bb06f6f8

webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp