This introduces masking of loads with speculation bit during code generation.
At the moment, this is done only under the
--branch-load-poisoning flag, and this CL enlarges the set of supported
platforms from {x64, arm, arm64} to {x64, arm, arm64, mips, mips64}.

Overview of changes:
- new register configuration configuration with one register reserved for
  the speculation poison/mask (kSpeculationPoisonRegister).
- in codegen, we introduce an update to the poison register at the starts
  of all successors of branches (and deopts) that are marked as safety
  branches (deopts).
- in memory optimizer, we lower all field and element loads to PoisonedLoads.
- poisoned loads are then masked in codegen with the poison register.
  (In this CL, this last step is left as a TODO. You can run with the flag,
   though mitigations will just not be effective at this time).
  * only integer loads are masked at the moment.

TBR=mstarzinger@chromium.org

Change-Id: Ie6eb8719bf85d49c03b4a28e2f054480195a1471
Reviewed-on: https://chromium-review.googlesource.com/973616
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52123}