Port b048c16b

Original Commit Message:

    The goal is to remove CL to remove the confusing implications for
    full poisoning.

    This is an alternative to
    https://chromium-review.googlesource.com/c/chromium/src/+/1253341
    where chrome has to work around our implication system.

    In the optimizing compiler, we already have a bottleneck for setting
    mitigation level in src/compiler/pipeline.cc, so it is easy to change
    back to partial mitigations.

R=jarin@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I96d0651eed2638abddb5486da1e2b55a84e97264
Reviewed-on: https://chromium-review.googlesource.com/c/1261797Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#56385}