~3-5% speed up on webrtc_perf_tests of vp9 on linux desktop. Avoid going thru a lot of unnecessary code checks. Change-Id: I2cb0d794bcf239c5057dfc04cd07a496f89a5016 Bug: None Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/167640 Commit-Queue: Jerome Jiang <jianj@google.com> Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Reviewed-by: Rasmus Brandt <brandtr@webrtc.org> Reviewed-by: Stefan Holmer <stefan@webrtc.org> Cr-Commit-Position: refs/heads/master@{#30586}