Better workaround for Bug 1089.

Don't initialize first pass costs for a number of symbols where first
pass probabilities aren't initialized.

This brings a 1.22x first pass speedup.

https://bugs.chromium.org/p/webm/issues/detail?id=1089

Change-Id: I97438c357bd88f52f5a15c697031cf0c3cc8f510
4 files changed